Think about the shape and size of your invitations in comparison with the style of wedding you are having. Formal weddings typically use a rectangular card for a more traditional look. However, if your wedding more on the playful side, think about going circular, scalloped, or even a square wedding invitation. Keep in mind, the bigger the envelope the more costly the postage.
Bottom line, you do not need to worry too much about not having too much money allocated for the most important and beautiful day of your life, because there have been and there still are many couples in this situation and almost all of them were able to have the wedding they have always wanted, and that because they knew how to spend their money intelligently. You can do that, too, as long as you have a good plan to follow and as long as you have a pretty good idea about how large or small your budget for the wedding is.