Fees or interest: Credit card

I frequently use Synchrony PayPal Credit Card for purchases over $140.00 that are supposed to be interest free if paid in full within six months. I rarely use the card for non-promotional purchases. I have been diligent about paying off these balances in full within the six month period so I am not charged back interest. I realized last month that I was being charged interest on nearly $3000.00 worth of purchases at an interest rate of 30 %. I asked customer service where this came from as I know I have been paying off these purchases before their expiration. The only answer I received was a series of billing statements in the mail. I believe Synchrony has purposefully allocated my payments incorrectly to charge me back interest and I still can not even figure out what was not paid off and what purchases I am being charged interest for. The online system is very difficult to navigate. I can see promo purchases and I have double checked to make sure they are getting paid off but I can not find any other information without sorting through statements. I truly believe that Synchrony has not applied my payments correctly in order to charge me hundreds in interest.