Finish: Your paper finish will affect how you're able to print and how much ink your invitation will take. Glossy papers take a lot more ink and longer to dry. Linen papers and papers with more texture have a much higher tendency to bleed, and so they won't work with some at_home printers where you can't change your ink settings. If you're not interested in researching finishes because you're not a weird nerd (like myself) who likes to feel on paper, a matte card stock is an easy, relatively foolproof option that will more than likely work for the printer you're using.
POSTAGE and Quantities: The number one tip you need when it comes to sending out your invites, is that your guest list number does not equal the number of invites your sending. Do the math on how many couples you have ahead of time, and you won't make the common mistake of ordering about double the invites you need.