I no longer had two day shipping to any address on my account. I could not initiate calls for customer care, the option disappeared for me, only chat was available. The return button disappeared for some purchases, my reviews disappeared for both products and sellers even though they showed up on my account. After a second return a few weeks later, I lost access to my kindle account, my associates account and my AWS console. Amazon customer care said access could not be reinstated and after that conversation, I could no longer place orders even though I could still log in and check order history.
Similiar things happened years later with my second account but on Amazon India, my product reviews disappeared and I could no longer leave seller ratings. I now have kindle and AWS on seperate accounts so I don't know if they were restricted for that period. My product reviews returned after about 18 months of no returns. This suggests there's an unspecified 'cooling off period' between returns within which account restrictions could be triggered by a return.
These days, I don't pursue returns unless if there's a glaring issue (completely wrong item sent, incorrect quantities, or outright seller fraud).