Product: Credit card or prepaid card

Hello, I opened this credit card account in   XXXX   XXXX  . The appealing factor of this card was the low annual interest rate and the ability to use a promotional no-interest ( until   XXXX   XXXX   ) balance transfer/convenience check option if done by  XX/XX/XXXX . I was able to utilize the use of several convenience checks by the deadline of XX/XX/XXXX  and that amount would not be subject to any interest until XX/XX/XXXX  ; with no interest due if paid off in full by that time.
I then used my credit card in the month of   XXXX   for 2 purchases ( one on   XXXX   and one on  XXXX  ). When I received my billing statement ending  XX/XX/XXXX , I noticed I have been charged interest for those amounts even though I had n't received a billing statement yet that included those charges.
I immediately inquired with the company to see if a billing error was the cause of this interest charge since I 'm used to seeing a grace period of 21 days to pay off the outstanding balance by the due date or be charged interest for that amount apply to the   XXXX   other USAA credit cards ( along with several other cards from   XXXX   XXXX  ,   XXXX   XXXX  ,  XXXX , etc. ) that I currently have. After several attempts to get a straight answer from the company with no resolution or call back I filed a complaint with the CFPB about this issue.
Several days later I received a call from someone within the USAA CEO 's office regarding this filed complaint. I returned her call and explained my issue once again. She appeared to understand the perspective that I had in which I was thinking the typical grace period would apply to purchases made, meaning if paid off before the due date then no interest would be charged. But she informed me that because I had a promotional balance ( the no interest offer until   XXXX   XXXX   ), the typical grade period did not apply to new purchases on this particular credit card, so interest would start to immediately be charged from the date of any recent purchases until the promotional balance was paid off. The grace period for this card would only apply once the entire promotional balance was paid off in full. I told her I did n't understand why this credit card was different that my other cards and she immediately told me the disclosures made this fact very clear and even quoted the section/part of the disclosure document. This immediately irritated my very much because she was basically saying it was my fault that I did n't understand the way this card worked while there was a promotional balance due. I told her I did not appreciate being told that, especially by someone whom had contacted me about a submitted CFPB complaint. She was unreceptive to my point of view and was constantly speaking over me. I told her I did not want to speak with her any further and was not satisfied with her reason given.
I feel this practice not having a traditional grade period applying when a consumer has a promotional balance is a violation of UDAAP because I am a relatively bright and engaged consumer and had no idea that this particular credit card had this 'querk ' until after the fact. This difference was not called out specifically to me because if it was I would have never used this credit card until the promotional balance had been completely paid off. I take full responsibility for not knowing this key fact, but also do not believe this was clearly and concisely laid out for me to fully understand.
I take great pride in paying off my monthly credit card balances in full each month, so why would I have used a credit card in which interest is going to be charged ( from the transaction date ) regardless if I pay it off or not when I have   XXXX   other credit cards that I can easily use that have the traditional grace period and allows me to pay the balance off in full with no interest being charged??
I am very disappointed in the response I received from USAA about my initial inquiry and am seriously considering moving all of my banking, assets and insurance services to another company after this issue.