I bought one of these for my phone about eight months ago just because I have a bad habit of sticking credit cards in my pockets on quick trips around the neighborhood and losing them. I'm happy to report that is still QUITE firmly on my phone and easily holding an ID, credit card, and sneaky $10 bill, which I especially love living in NYC. Instead of making a whole production every time I leave now, I can just grab my keys, my hybrid phone/wallet, and mask, and be right out the door.
Promising review: "Has completely replaced my wallet! I pretty much only need three cards in my day to day life — my credit card, my ID, and my public transit card. This sleeve holds all three perfectly without ever losing a single one, and has held up really well despite how careless I am with my things, so I haven't carried a wallet in months. It's pretty tight at first but will stretch to hold your cards as needed — note that this means that if you take a card out, it will be fairly loose for the others, so be careful. I got the pink color and it's picked up a little color around the edges from my jeans, but otherwise has remained pretty much the same color. I highly recommend this if you're looking for a stick-on wallet sleeve for your phone!" —Alexis
Get it from Amazon for $13.95 (available in 12 colors).