What should I watch out for after staking a student loan through the heart?

I have been paying it down in chunks until it was under 1k, and figured that I might as well finish it off. So yesterday I wrote a check for that amount and mailed it, rather than let the loan holder milk the account for interest for a few more months.

Now I’m wondering what kind of grief I should be expecting from them. Any ideas?
I have already identified two potential issues:
1) I put the check in the mail on the 12th, and the deadline is the 14th, and they’re in New York and I’m in Missouri. I’m hoping that the postmark will be enough to settle the matter.
2) The last payment chunk was split between interest and principle, and I had up until now paying the interest through automatic withdrawal. I fear that they’ll take the payment and apply part to the principle, and part against future interest accrual.