In my panniers, Is where the pouch with a Zipper is . which does wallet function.
It has a caribiner clip on it.. I have a ring to clip it onto,
I have other pouches for the Checkbook , some folding Reading glasses,
+ a case with sunglasses in them. each clips to the ring.

On my Bike tours I wore a Money-belt under my clothes (& raingear).
Ortlieb makes a waterproof one of those.. too..