Would it be possible to do the tip as a calculated shipping rate?
For e.g. what if I added weight to all of my items. The weight in pounds would equal to the dollar amount to calculate the tip. See screenshot below, I don't know if this would work since I have a trial version at the moment.
Basically what it would be is:
Food Item = $10.00 , set the weight of the item to be Weight = 10 Pounds, so the tip would be $1.50 (based on 15%)
Food Item = 20.00 , set the weifht of the item to be Weight = 20 Pounds, sot the tip would be $3.00 (based on 15%)