Hey neilo,
You just need to think outside the box and change your client terms to match the new terms for the developer. Change all of your Submit buttons to PayPal Buy Now buttons. Put a message on the forms explaining the new Developer's Tax and the amount of the fee for using the form.
You should be able to recover your investment in no time, as long as your client's visitors go along with the new terms of use. Don't forget to change the WULA (Website User's License Agreement) to cover the new charges.
BTW, thanks for the reminder of the other solution. I forgot I had that.
