Bubbles are tonnes easier than card, no reason to be worried really with this - use Goo gone if you have some, if not try a baby wipe, slowly do circular motions over the sticker till it starts coming up - you could try with with water if you want - just don't get it on the card.
Once the sticker is off you might be left with some residue- just use your nail to pick it off and use the wipe to clean the rest.
Just be careful with the amount of pressure you put on the bubble as you don't want to ding it, but you can't do too much damage if you take your time.