Bests & Worsts Reviews from Amazon

I have been paying the balance on my Amazon Store card sometimes a little early before the monthly statement has been generated. But since I mostly use this card for my "subscribe and save" items I know the balance will not change. I recently discovered that if you pay your full balance prior to the system generating the monthly statement, you will be charged a "monthly interest charge". This makes absolutely no sense to me at all. The only solution the Synchrony customer service rep suggested was to not pay my balance until the monthly statement has been generated and they refuse to reverse those "interest" charges that they have been adding to my balance for several months. Make sure you check your statement if you do happen to make payments early. I can understand being penalized for late payments but this is totally crazy to be charged for "interest" for paying early. What agency should I file my complaints to?