I recently ordered my invitations and found it very helpful to have the text written/typed out and a sample with a layout sketched to give them an idea of what I wanted. The stationer was very appreciative that I made the effort to make her job easier, and she was able to set up everything like we wanted where we were very satisfied with the layout except to change the proportions of certain things. If you're having your invitations designed, if you see something that inspires you, it's well worth the trouble of clipping the image you liked to provide a guideline for the graphic artist or making a basic sketch if you have an idea of how you want things placed.
Whoever said it was supposed to be happily ever after is a big fat liar.