In my experience they've always required me to go to a UPS store or retail location (Kohl's, Whole Foods, etc.) to process a return, even when it's a counterfeit product.
Returns I've elected are a great process; being forced to run errands for a $trillion+ company because they've deceived you about the bedsheets they are selling is something else entirely.
There's a point to be made about how it's not possible for Amazon to check every single listing (made by other companies) for authenticity, nor they can use user feedback as a way to check/block those companies as many people use a different return reason than the real one (eg. "I don't like it" becomes "it's counterfeit").
It's unfortunate but you'll have to learn how to navigate the new amazon, and possibly use the mentality that everything CAN be counterfeit on this platform.
I think this is definitely true - and it's enough that I just don't find it worthwhile to use the service anymore. The overhead of doing detective work to determine whether something might be counterfeit, and the too-commonly-realized risk that I'll need to re-pack something and schlep it to UPS makes it utterly not worthwhile.
One will basically never encounter this problem with, e.g., Target, Best Buy, NewEgg, Ikea, Bed, Bath, & Beyond, or other retailers - what makes Amazon preferable to any of those brands, when the diligence you described is required to shop there and not elsewhere?
Returns I've elected are a great process; being forced to run errands for a $trillion+ company because they've deceived you about the bedsheets they are selling is something else entirely.