Hi!
I might be answering this a little late, but I want to still post this for anyone else out there who is considering making their own invitations.
It took me a long time to find papers I liked in bulk. So first of all i will give you a few websites to check out:
http://lcipaper.com/ They have some cute papers and neat invite ideas, although it will cost the same (and if you get even a little fancy- more) than ordering them up.
I had also thought I wanted to have white lettering printed on my invites at first because I wanted a fall wedding with dark color invitations, but ended bumping my wedding date to summer time because my soon to be father in law is very ill. So I found a printing company in my area (http://www.minutemanpress.com/) and learned that I can pick out my paper weight, color and font color and have them cut everything to size for cheaper than I could buy or cut it for myself. I order 2 sets of 350 5"x7" printed on pieces of paper, and they made me a map to go with it for $103! no joke. I was so thrilled! But I took mine to the next level... I found a seal and send basic white invitation from a company that makes this stuff in bulk (check out the 'self mailers' under the specialty section at http://doubleupaper.com/). You can order 250 of these things, for $32 (remember to add in shipping and handling though). These replaced me having to buy my envelopes and RSVPs. When I got them I printed the RSVP info on the perforated card at the bottom and then used double stick tape to attatch my two 5"x7" cards on the available spaces (one for ceremony, one for reception). To make it even more special I added a sheer ribbon across the top of my upper 5x7 because I placed them all in there landscape. overall my invites cost me somewhere around 70 cents each!
It's easier to assemble invitations like this if you have an assembly line. Gather a bumch of friends and family to help you out. Make it a fun afternoon by ending with drinks or games if you'd like.
Other advice:
Remember that square shaped invitations cost extra postage because they have to hand sort them.
Also, if you use a metal brad or some other metal charm on your card or have a large knot from a bow, that can also cost extra postage because it won't go through the machine.
Remember there are also weight requirements, so don't use all cardstock if you plan on layering your invites... and just to make sure whip up a practice one, bring it in and get it weighed at your post office.
A postcard RSVP can be just as elegant and easier for your guest to use and also the postage is almost half that of a card in an envelope.
Most importantly- shop around! If you find a paper you like, find the company that makes it and see what other retailers they have! It does take time, but do your homework and you'll save a bundle!
Have fun! Any questions, email: fire_e_hott@hotmail.com