Whilst taking a payment in POS module, we have noticed something different than if you Take Payment from an Invoice.
If using card you have to manually key in the amount (unlike via Invoice method which automatically puts it in). This opens up opportunity for errors in keying in incorrectly.

So, with a card, have it automatically put in the full amount.

Also, with Cash. Have buttons that can be clicked for common note demominations such as 5, 10, 20, 50 etc. Again just to streamline and reduce any error opportunities.