I see you added the total/paid/balance to the payment entry. Thank you for that.

One change for that. You used sale->total which does not include order tax. I changed it in my install to use sale->grand_total. I have a vague idea that could be done conditionally, total or grand_total, based on how tax is set up.

Second, to the point of this request: I also added a change calculator. See the attached image.

I put this below the Note as we require a note on each payment to denote cash or charge or check. For most users I imagine having it above the note is better.

The fields are not saved or processed. They are simply there to do the math for those who aren’t good with math.

As you might guess, we use SIM in almost a POS fashion. We don’t use inventory, so SIM is easier than even your Simple POS app. If Simple POS had a “no inventory” mode, I might use it instead. But, SIM is fine.