I just tried GrubHub and the "Driver Tip" defaults to 20% which gets calculated as 3.27 for my order having a subtotal of 13.95, waived delivery fee due to Amazon prime integration, 1.40 service fee, and 1.01 tax.

When adding up 13.95+1.40+1.01, and then taking 20%, it comes to 3.27

Totally bogus. Well, reasonable at this subtotal, but not at higher subtotals... Because using a percentage is nonsensical in the first place.