I think business people such as cab drivers should be prepared for twenties. Anything bigger than that is iffy.
For some reason, when I work the church rummage sale, the first person every year is buying $2 with a $100 bill. It's not even that easy to *get* $100 bills in Canada.
I suspect at least a few cab drivers, and rummage sale purchasers, expect you to "let it go." Give the driver his $13 tip, and give the person at the sale the $2 for free, because hey, it's just two bucks, right?