Does anyone know what happens if you pay after the Sept 1st deadline (Sept 2nd to be exact)? Is it just a $50 late fee? Or are there other consequences?

You likely start to accrue interest. Not sure what else they can possibly do to you.

how much interest can accumulate over 1 day? o.o

For every 100$ you spend you have to pay 1$ the first day, 1.01 the next day... And loans are typically larger and typically you don't pay it off in 1 day.