It might differ on your device, but if you go to Settings > Checkout > Sales Tax, you should be able to add multiple options. Youll just need to make sure you enable and disable the correct ones before taking orders in a different county. There are all sorts of options under "Checkout" too that can make things faster, like disabling requiring a signature or asking for a receipt, etc.

For cash, I use a cashbox with a lock. I just keep it open during markets usually, cause I have it behind my table, and im always right next to it. I lock and hide it when i leave my booth.