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.
Flat unfolded cards and traditional card blanks are great if you’re on a budget or don’t have huge amounts of information to include. Wallets are great for including lots of extra information inserts, RSVP cards as well as the main invitations themselves. There are also in_between options that give you the freedom of a wallet on the budget of a traditional card.