S.B.
If you go to an office supply store, or a craft store, you can find kits that include the blank cards and envelopes. Usually, the company that makes the kits has a website you can go to and design your invitation, then print it from the website.
I recently made invites for my son's 1st birthday. I used powerpoint, and made a slide with 4 invites on it, then just made multiple copies of that slide, printed, and cut them apart. They were about the size of a postcard. Then I just bought colorful envelopes from the craft store to send them out in. I like this method, because it allows me to do whatever I want in terms of the design, layout, font, pictures, etc. I use google and look for pictures, them copy them into the slide and adjust the size as needed.
I also like the idea of using something like snapfish and using your daughters picture on the invite.