The elastic is extremely durable and we were not able to stretch out the elastic even when we tried under extreme circumstances. We have both been using our prototypes for 4-5 months so far with no signs of wear on the elastic.
The "T" design of the track stops your cards from being able to slide out of the bottom and the elastic snugly holds the cards so they aren't able to fall out. In a shake test, holding the wallet upside down with one card in it, I was unable to "shake" the card out.
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