Student loan irritation

I have a parent's Federal PLUS Loan with the ACS (formerly College Loan Corporation). I would like to start making an extra payment or two without changing the amount of my monthly automatic withdrawal. The web site that ACS does not say anything about extra payments. After contacting their help team via email twice, I get the idea that if I make my extra payment on a certain date, it will go in as "extra" - so today's the day. I log on to their web site. I can see my account details. I can see that there's a place that indicates the regular amount of my monthly payment I'm making to them. There's a slot for filling in a new amount - but once again, it's not clear that it's going to count as an extra payment. I only have the say-so of their help team. There's no indication on their web form about extra payments at all. They want me to put in my banking information again. There's no place to check "make an extra payment and use the bank info you have on file for me." Will the extra payment reduce the principal of the loan? Can't tell. Oh - and how long will it take me to pay off the loan at the current rate? Can't tell. If I make an extra payment, how will that affect the length of time it will take to pay off the loan? Can't tell. I guess I get to do the math? Totally annoying. I'll keep trying to track down enough info to make sure that I'd be doing an extra payment correctly, but I'm not up to tackling that today.