I just copied and pasted the coupon code from the active offers page and it worked. It did ask for my credit card security number (since I haven't done an online purchase in a while) but took the $10 off immediately.
I just did it like you mentioned, and it also asks for my credit card info.
when I put all the details I get "transaction declined" I already tried 2 different cards.
Im trying to purchase a $10 PSN card with the animal crossing $10 certificate but I always get transaction declined