If you put about 4 cards in it, or a couple of cards and some cash there is enough pressure inside the wallet to hold everything in and you can turn it upside down and shake it and nothing falls out. I have a gif on my twitter account. This is printed in NinjaFlex and uses about 20 grams of filament. The flexibility allows you to easily retrieve your cards.
I made one for my son and he wanted to be able to put his cash in like you do for a wallet so I made it taller.. cash_wallet
Even though this is not printed in a flexible filament I'd like to let you know I create these designs to encourage people to try out flexible filaments or get some new interesting colors of flexibles. For more info on purchasing starter quantities see flexyfilament.com