If code was related to a [tag:payment-system] then I would agree that the tag was useful.

Credit cards are not a payment system though, so none of those questions would qualify.

[tag:payment-system] would be things like Moneris, EFT, Paypal, etc. I would expect the tag to relate to the Wikipedia entry at least a little bit:

> The payment system is an operational network - governed by laws, rules and standards - that links bank accounts and provides the functionality for monetary exchange using bank deposits

I don't believe any Code Review questions referenced in the question have anything to do with payment systems, other than the following:

 - http://codereview.stackexchange.com/questions/38953/thread-safe-wrapper-around-wcf-servicemodel-clientbase-subclass
 - http://codereview.stackexchange.com/questions/27755/secure-curl-to-handle-response-from-payment-gateway

The rest have to do with [tag:shopping-cart] and [tag:validation].

I don't believe the questions above have enough content specifically related to payment systems to make the tag worthwhile.