There is no law saying that a business cannot charge customers for additional amounts or services later, if it discovers that it had previously failed to bill them for everything. There are going to be specific cases when they might not be able to, such as--the debt is too old to enforce, since the statute of limitations has passed;the employer had previously sent an invoice or bill stating that paying the indicated amount would be "payment in full" of all amounts outstanding or of the account--in that case, if the customer paid pursuant to that bill or invoice, it might be held that the employer contractually gave up the right to charge more in relation to those services--but other than that, the business should be able to do this.