I've thought about using them like that as someone without access to privacy.com, but they do charge an activation fee and other random little fees I didn't want to deal with. So I just... didn't buy whatever it was I was considering at the time lol. Always keeping an eye open to see if there are any alternatives though.

In the US, gift cards will often be declined for setting up ongoing transactions. Every transaction has a merchant code associated with it and many subscriptions and services will read the merchant code and reject it on that basis alone. Doesn't matter how much of a balance the card carries.