Additional Money Sent in Never Gets Posted to Lower my Principal Balance.

Forgot to mention: They're (Sartander) known to charge early repayment
fees. Might be a good idea to re-read the contract.

It's Sartander! 0% chance it's by mistake. Best thing you can do is
send a second check mid-cycle and hope it limits their ability to play
stupid.

I experienced the exact, same issue with Sallie-Mae and had to call
monthly requesting an adjustment. I tried writing something like
"Apply excess to principle" in the memo line with no success. Next I
tried using pre-printed stickers with account number, loan name/group
number, and apply to principle with no success. It still took calling
once per month forcing them to do an adjustment. Sallie-Mae's system
kept paying me forward instead of applying to principle.

The best way is to send a separate check for this purpose. On the
memo field Write "APPLY to PRINCIPAL loan ID xxxxxx". You can send
both checks together and it should applied to principal on the same
day.

Perhaps two separate checks will do the trick. One for payment, the
other for principle. This also give proof of your request.