You disregard privacy policy of organizations you deal with. You have no audit reports of amazon or any other retailer to know what they do with your private and purchase information. Since you don’t have that, you are only hypothesising that 3rd party platforms will keep you safe.
Therefore, arrangement between mullvad and their resellers are questionable.
Like I said, you do you.
Assuming someone already has an amazon account so they have a name, address, and credit card on file, when it comes to the VPN, that purchase history is all they'd get right? That'd be the same as everything you buy. If I use amazon to buy a toaster, they know I bought a toaster. They don't know if I give it to someone as a gift, or what times I used that toaster, or if I ever use it at all, or what I do with the toaster I bought.
Assuming Amazon sells your purchase history to a data broker it would show them that you paid for a VPN service, but not if you ever used it, or when you used it, or what you used it for. If you live somewhere where using any kind of VPN at all is illegal I suppose that would bad, but otherwise is getting flagged as "has paid for and maybe used a VPN server" something that can make you unsafe? Is it any more/less safe than being flagged as "has bought and maybe used a toaster"?
I've seen people suggest similar scratch off cards as age verification, and that has real security issues because the cards are to be used at websites who shouldn't have any idea what your identity is. The website knows what IP used which code, at what time though and so it might be possible to link the use of a code to the place where it was purchased, and identify the person who used it that way. When pre-paid calling cards were a big thing this is how they tracked down people who abused them. That's not an issue for VPN scratch off cards though, because the codes are only used at the VPN provider's website. Anyone not using the cards is already giving that website their identity, so using the cards would just be mean there are extra steps to figuring out who paid for them.