We ordered our save the dates from VistaPrint. We did postcards, which I think the minimum number you can buy might be higher than you want, but we ordered 50 for about $20.
For our invitations, we ordered the design from an Etsy seller and then had them printed by VistaPrint. For their invitations, I think the minimum is 10 or 20. It was very inexpensive, and the quality of the invitations when we got them was great!
If you use VistaPrint, definitely wait until they have a 40-50% off sale if you can. We really tried hard not to spend a lot on the invitations, and I think despite the small budget we had, they came out super nice!