I have created a form with several fields that accumulate to a total charge, which is passed on to PayPal. This works fine. But I would like to supply some customers with a discount code to enter in a discount code field that results in zero charge, because they are being invoiced offline.

So I have a discount code field and a product field with a calculated negative amount (adding up all the other charges and multiplying by -1), with conditional logic only to turn this on when the correct discount code appears in the discount code field. I have disabled the quantity field. The calculation formula is:

( {Would you like a standard vendor sponsorship or a Premium Vendor Sponsorship? :14} + {How many 10' x 10' spaces would you like?:15} +{Do you need electricity:8} +{Do you need to rent tents from us?:9} + {Do you need to rent tables and chairs from us?:10} ) * ( -1 )

When the form is filled in, this field is calculating and showing a negative value that equals the total of the fields with charges, and thus, a zero net charge. But, when clicking Submit, the form goes to PayPal and charges the full amount without the calculated discount. I would like instead for Submit to complete the process because the resulting charge should be zero.

When I change the formula to leave out one of the values, then the calculation produces a net positive price, and this is reflected correctly on PayPal. For example if I delete the tables and chairs value from the formula, PayPal shows all the charges, a discount for everything but the tables and chairs, and a net charge for just the tables and chairs.

So, how can I get a 100% discount to work in this situation?

(Temporary URL: http://www.slowlivingsummit.org/soth/slow-living-expo-registration/

Throwaway discount code TESTING123)