I don't mean to sound harsh when I say this, but opening someone elses mail is a big no no, federal offense and all that jazz. You two are not married anymore correct? If this credit card is his, and his alone, it's not your business anymore.
Second.. not your job to forward his mail either, if he's too damn lazy to change the address well, "return to sender", "doesn't live here anymore" "not at this address" and stick it back in the mail box, or return it to the post office. Sending it on to him with a little note is codie behavior hon.
I did this at first with my non alcoholic ex-assclown myself after my divorce for awhile.. so I'm guilty of this as well. I got some great advice from my abuse group.. basically it said.. it's not my problem, and it's not my responsibility. It isn't yours either.