I have sent a note in with the payment before, that says "apply the additional funds to the principal only." That was before I did everything on Bill Pay though. Now I just put that in on my Bill Pay Memo, and it seems to work.
Good luck!
Having just paid off $18,000 in credit card debt I feel your pain. We sent in every extra penny we could afford and got it paid off in 2.5 years.
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“All his life he tried to be a good person. Many times, however, he failed.
For after all, he was only human. He wasn't a dog.”
― Charles M. Schultz
Warning: The above post may be dripping in sarcasm and full of smartassedness.