Everyone gets one chance to choose to go to the end, and when they use it, they get a warning that I don't work like that. Someone who uses two chances doesn't get invited back the next year. 

We've used a "your fee will be $X; if all of the information within your control is in by March 1, you get a 50% discount" carrot, where $X is twice the prior-year fee.

The only one I've got still hanging out there is a DocuSign for someone who took extra time (at my suggestion) to decide whether to apply or refund the overpayment, based on whether a contingency occurred.