Weight: Paper thickness is measured by weight. The higher number you see, in pounds, on the packaging label, the thicker your paper is. This doesn't mean you have to find the thickest paper you can—most card stocks are a great weight for invitations and can hold up to being mailed.
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.