I own a business. Sometimes I fuck up in favor of the customer. When this happens, I would never even dream of asking them to pay me back for my mistake or otherwise make it right. I've never worked anywhere that would, either.
I could understand a business going "Hey we screwed up, if you're going to use that, any chance you could pay for it? If not, no big deal, it was our mistake," even if I personally would never do that.
But telling a customer they have to do it? To me, that's a fundamental shift in attitude, especially when there is an implicit threat to not send them all the other stuff they paid for if you don't comply. How are we supposed to know that by paying up we're not encouraging that sort of behavior?
In a normal situation, on the customer end, if they give me a return shipping label and arrange pickup, I'm happy to send it back. Not in this sort of situation, and I'm sure going to push back way fucking hard on them trying to get me to pay up for their mistake, and if I get forced to pay because they're holding the rest of my shit hostage, I'm never purchasing from them ever again.