I don't charge extra -------------- I just grumble a lot about those people that wait until the last minute.

I like people but I don't like grumbling. People are basically good. So as I get them to get me their stuff earlier, I will grumble less and like my clients more.

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.

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.

I like it.

I try to remember how grateful I was on April 15 that at least I had a little less work because some of the procrastinators were filing extensions.  My fees are based on time and effort needed, and there is more of that involved six months after getting out of the routine of finishing several returns every day.  But if I wanted to add a line item to an itemized bill, how would this work?  If there is a balance due, 2.5% of the amount, since you're saving the client 5% by making sure they're not a day late.  If there is a refund, 3.5% of the amount, since that's how much interest IRS will pay if they don't send the refund until December.  (For credit-elect, that's how much 2210 penalty is avoided.)  

This is what I'm planning to do as well.

Several years ago, I charged 10% of the fee for data received after July for 1040s, 20% after August for 1040s, 30% after September.

Corporations were 20%, 25% and 30% for the same time limits.

I only got a few, but none of them did it again.