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.
Then order lots of samples, as you might totally change your mind when you see them, or suddenly be inspired by the best bits of different samples. The Wedding Crafter's 'Design your own wedding invitations kit' is a pack designed to give beginners chance to have a play with different card shapes, inserts, ribbons and embellishments that can be tailored to suit any wedding colour scheme.