Include explanation "Payment will be made today if you don't select a different date".
You have a new feature that allows payers to selected a different date. I looked at that and wondered: "if I don't select a date, will payment be made today?" Or... "if I don't selected a date - will payment never be made?"
It's a good option - remember the old adage about overwhelming out day with small decisions: we need to save our decision-making energy for the important stuff.
So please include advice to the effect:
"New Features: click here to select a different payment date"
"note: payment will be made today if you don't select a date".