Hint...if you pay it Sunday its still on time .... seriously Jcarrera?
"I just have no idea how this is even an issue?"
All of the "workarounds" posted never address the point of why do you allow your billing computer to set a Sunday for a due date in the first place.
Snail mail, as suggested in the previous post will not get there on a Sunday--nor soon, on a Saturday either!
And some other payment methods do not "work" on a Sunday (either are not delivered, or are not posted on a Sunday).
You are totally and completely missing the point.
That there are methods to "get it there on time" was not the issue at all. I grant that; NOT THE POINT.
The issue was a billing system, which in some cases, unnecessarily shortened the customer's payment window by a day, and which is preventable by "correct" programming. Program it so that if the computed day falls on a Sunday or a holiday, simply add one day (computers can do that pretty easy)--then check to see if THAT is a Sunday or a holiday; if so, repeat.