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.
Start with the basic invitation _ There are loads of different styles and types of cards, depending on your budget and how much information you need to include. When you order your samples, try to make sure you have some of each style to see what would work practically for you.