If you would like users to enter a coupon code in a form and get a discount depending on the code they entered, use this code example. In order for this code to work, you will need to create a new form and add a single line text field for the discount amount. Submit an entry for a discount amount. The coupon code will go in the entry key box. Submit a new entry for each coupon code.

This will replace the default invoice number with the value from a field in your form.