deemed independent under the rules and regulations of the Securities and Exchange Commission or the Public Company Accounting Oversight Board. The parties have agreed to cooperate with each other and use reasonable best efforts to promptly take such actions to cause the closing conditions to be satisfied as promptly as reasonably practicable.
2019 compared to 2018 - Medicare approved members grew 81% in 2019 compared to 2018. The growth was primarily due to an 88% growth in Medicare Advantage submitted applications and an 84% growth in Medicare
Estimated membership represents the estimated number of members active as of the date indicated based on the number of members for whom we have received or applied a commission payment during the period of estimation. The following table shows estimated membership by product as of the periods presented below:
2020 compared to 2019 - Customer care and enrollment expenses increased by $38.6 million, or 29%, in 2020 compared to 2019. This increase was primarily driven by $27.5 million increase in personnel costs associated with an increase in customer care and enrollment headcount, $5.1 million increase in consulting expenses, $2.8 million increase in facilities and other operating expenses, $1.3 million increase in stock-based compensation and $1.0 million increase in licensing costs.
overtime costs and average call length during the peak volume period in the fourth quarter, as well as the expenses incurred related to the opening of the new customer care center in Indianapolis.
As of December 31, 2020 and 2019, we had 1.1 million and 1.0 million shares held in treasury stock, respectively, that were shares repurchased to satisfy tax withholding obligations. As of December 31, 2020 and 2019, we had a total of 11.8 million and 11.6 million shares held in treasury stock, respectively, including 10.7 million shares previously repurchased.
As of December 31, 2020, we did not have any off-balance sheet arrangements, as defined in Item 303(a)(4)(ii) of Regulation S-K, that have or are reasonably likely to have a current or future effect on our financial condition, changes in our financial condition, revenue, or expenses, results of operations, liquidity, capital expenditures, or capital resources that is material to investors.
Operating Segments - We report segment information based on how our chief executive officer, who is our chief operating decision maker (“CODM”), regularly reviews our operating results, allocates resources and makes decisions regarding our business operations. The performance measures of our segments include total revenue and profit (loss). Our business structure is comprised of two operating segments:
Revenue Recognition - Our commission revenue consists of commission payments from health insurance carriers whose health insurance policies are purchased through our ecommerce platforms or telephonically via our customer care center, and bonus payments which are generally based on our attaining predetermined target sales levels or other objectives, as determined by the health insurance carriers. In addition, we also generate revenue from non-commission sources, which include online sponsorship, advertising, lead referrals, and technology licensing.
purchase individual and family health insurance outside of these open enrollment periods, unless they qualify for a special enrollment period as a result of certain qualifying events, such as losing employer-sponsored health insurance or moving to another state due to the recent COVID-19 pandemic.
We will continue to monitor our member retention rates as compared to our forecasts and other market factors and evaluate whether any addition or reduction of adjustment revenue shall be recorded as we continue to assess our LTV models in future periods.
The fair values of the acquired technology were determined by using income and cost methods. Fair value of trade names were determined using the profit allocation method, which is based on the estimated royalties we are relieved from paying because we own the assets.
We are amortizing the existing technology, trade names, trademarks, and website addresses using the straight-line method over an estimated life of 3 and 10 years, respectively. The estimated useful lives are based on the time periods during which the intangibles are expected to result in incremental cash flows. Acquisition-related costs incurred were immaterial during the year ended December 31, 2018.
During the years ended December 31, 2020, 2019, and 2018, amortization expense related to intangible assets totaled $1.5 million, $2.2 million, and $2.1 million, respectively.
Our cash equivalents were invested in money market funds and commercial paper with original maturity of 90 days or less were classified as Level 1. We endeavor to utilize the best available information in measuring fair value. We used observable prices in active markets in determining the classification of our money market funds as Level 1. There were no transfers between the hierarchy levels during the years ended December 31, 2020 and 2019.
Common Stock - On all matters submitted to our stockholders for vote, our common stockholders are entitled to one vote per share, voting together as a single class, and do not have cumulative voting rights. Accordingly, the holders of a majority of the shares of common stock entitled to vote in any election of directors can elect all of the directors standing for election, if they so choose. Subject to preferences that may apply to any shares of preferred stock outstanding, the holders of common stock are entitled to share equally in any dividends, when and if declared by our board of directors. Upon the occurrence of a liquidation, dissolution or winding-up, the holders of common stock are entitled to share equally in all assets remaining after the payment of any liabilities and the liquidation preferences on any outstanding preferred stock. Holders of common stock have no preemptive or conversion rights or other subscription rights and there are no redemption or sinking funds provisions applicable to the common stock.
Stock-Based Compensation Expense - The fair value of stock options granted to employees was estimated using the Black-Scholes option-pricing model and with the following weighted average assumptions for the years presented below, except for 2020 in which we did not have any options granted:
accordance with the settlement agreement and the court order approving the settlement agreement, the parties will seek a dismissal with prejudice of the Gonzalez Complaint.
Substantially all revenue for the years ended December 31, 2020, 2019 and 2018 was generated from customers located in the United States. Carriers representing 10% or more of our total revenue are summarized as follows:
On December 20, 2019, we amended our revolving credit facility agreement with RBC (the “Amendment”) and increased the borrowing amount from $40.0 to $75.0 million. The maturity date has been extended to December 20, 2022.
We incurred $1.2 million of issuance costs in connection with the Credit Agreement, which were capitalized as part of Other assets on our Consolidated Balance Sheet in the period we entered into the Credit Agreement. The Amendment did not change the interest rate. In connection with this Amendment, we incurred closing costs totaling $0.5 million, which were capitalized and recorded as Other assets on our Consolidated Balance Sheet as of December 31, 2020. The remaining balance of unamortized issuance costs was $0.7 million and $1.1 million as of December 31, 2020 and 2019, respectively.
The effective tax rate of our provision for income taxes differs from the federal statutory rate as follows:
The net operating loss and tax credit carryforwards as of December 31, 2020 are summarized as follows (in thousands):
the CARES Act did not have a material impact on our Consolidated Financial Statements for the year ended December 31, 2020.
On February 17, 2021,we entered into an investment agreement with an affiliate of H.I.G. Capital (together with its affiliated funds, “H.I.G.”), pursuant to which we have agreed to issue and sell to affiliated entities of H.I.G., 2,250,000 shares of the Company’s newly designated Series A preferred stock for an aggregate purchase price of $225.0 million.