I'm using Visitaprint for my printed items and designing a "postcard". The postcard options is like $35 for 100, if you select Wedding invitation it's almost $200 for 70. If you don't like their design options go to Canva and upload the file.
I got the Celebrate It invites from Micheal's. Same idea as the Gartner invites. Mine were on a clearance rack I got 2 boxes of 30 for $10/box. They are plain white but I'll use dark purple for the lettering. Instead of using the ribbons that came with it I may get some sparkly paper and cut it up to make a closure for them. I think putting a knot in them might make them too wide for standard mail...
Check out Gartner Studios. You can buy the box sets at Michaels of 50 invites, 50 envelopes 50 response cards and 50 envelopes for the response cards for around $25. You just take the box set and there's a code on the box that you use when you go onto their website and it gives you the template for that box. You can customize the lettering, font and color and you can write whatever you would like and print them right from home!
My fiance and I are using this for our invites and we had also used them for our save the dates as well. They turned out great!