“Fulfilled by Amazon” just means it was stored in their warehouse for a fee. It doesn’t guarantee anything else.
My experience is that if a 'fulfilled by' product needs returned, there's absolutely no hassle or delay in doing so.
I drop a return off at UPS in town and the funds are back in my account by the time I get back to the house.
It only took a couple sour 3rd party transactions to figure that out.