This has probably been asked before but I couldn't find the answer in a search.
Is there any way, using any of the accepted payment systems, to have the form data only be processed after a PayPal or other payment was completed? As is, there is an extra step in bookkeeping to verify that payments were made, because the form data is submitted regardless. I believe I heard mention that the payment button could be first with a redirect back to the form page, but this does not insert the payment info into the form's data fields.
If Shopping Cart was responsive.. and allowed for custom client information fields... it would be an excellent solution as I believe it will not process the data until payment is verified.

Something that would help with double checking the database with PayPal to make sure payments were processed:
If a record number, like a random number, could be sent with the form and also attached to the PayPal transaction record, so that there was a definite connection between the two. As it is now, The person who pays at Paypal (name on card, etc) may not be the same person who fills out the form, which makes cross checking difficult.

This isn't that easy and depends on the need. It comes down to having an api process the transaction and then allow the user to enter a code to get access to the form.