Curious question that I just thought about for anyone else that may want to know. How does one go about prorating payments? I know how to do the math, but does WHMAP, ModernBill or any other billing software that have this option?

Modernbill has the option

Currently, WHMAP does not. I forget exactly, but I believe the new version, version 3, of WHM Auto Pilot will have it as a feature.