Choose invitations that actually look inviting. Thick, high quality paper with bright colors or a unique design that makes the envelope stand out from other mail that may arrive on the same day. Your invitations should excite interest even before they are opened, and if you have chosen your design well, they should evoke reminders of the happy couple as well!
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.