I like wallets and have a collection gathering pace. They're fun and a lot cheaper than watches!
Anyway, as there many ways different styles of wallets handle cash, the move to polymer notes means that my notes often have creases from having been folded in half or thirds. Even new notes often curl up making them annoying to fold, or count.
I investigated some on-line suggestions how to flatten them.
1. Placing notes between heavy books and placing weights on top. This didn't really work with the number of notes I had. There was simply not enough pressure.
2. Ironing notes on low heat under a cotton shirt. This was better than the above option but any sane person hates ironing at the best of times, and there's the risk of melting the notes if you lose attention.
3. Placing notes in a waterproof ziplock bag and then dunking the bag in boiling water. This actually works.
As I understand it the heat "relaxes" the polymers which have been stressed when previously folded, or if the note curls without being folded.
I put three well folded tenners into a ziplock, removed as much air as possible, and put the bag into a shallow bowl of just boiled water. I held the ziplock under the water for a minute with some tongs and then checked the results.
Once the notes were cool, they laid flat. You could tell that they'd been folded previously but they were essentially flat.
Hopefully this small hack, may help if you find crinkled or unwieldy notes annoying and need to count them or fold them easily to your liking.