Charge a total calculated with logic

1. In Logic → Variables, add a number variable such as total.
2. Add rules that Add to or Set total from the answers (see calculate a total).
3. On the Payment question, set Amount to From logic and pick total under Charge the value of.

The variable is in whole currency units: 24.5 is charged as 24.50. If the total is 0, there's nothing to pay and the form moves on.