You can contact customer service, but unless you want the song in the same key as the one you accidently printed--you're pretty much going to be stuck with it. It's your responsibility to check and make sure you have exactly the version you want before you print it out. Except for certain arrangements that the company does in-house, you only paid for one copy. The website is awfully good about letting you reprint if something goes wrong with your printer (paper jams, running out of ink, etc). However, you shouldn't abuse the privilege if they do let you reprint something.
Hey, I've done the same thing as you a COUPLE of times because while testing different keys, forgot to switch back to the one I really wanted. I just keep it and deal with it (because while it might not be the ideal key--it's usually not something completely out of my range). I've deliberately bought songs in two different keys as well---but I had to pay for each different key.
I don't think you can go to a regular music store, buy the wrong sheet music, keep it, and then expect the store to give you a different version for free. Of course, with the store, you have a returnable physical copy (which still may not be acccepted back if you leave the store with it). With an online service, you get a copy that prints out on your own computer (so quality can vary with computers and printers), plus a digital copy that you can't exactly send back either.
Makes sense? Just be super careful in the future.
Note to Franz: Musicnotes.com only shows one sample page for their songs. You know--so people can't just print off the whole thing for free?
Personal experience (I buy a stupid amount of sheet music from various sources including musicnotes.com)