If you have a membership to Costco, it's a great resource for artist who want to sell cards of their paintings. I used it to make a batch of cards that I can sell during the holidays as well as at the Fremont Art Association. It's very simple and the quality of the prints are pretty good.
Go to Costco Photo Center and create an account. I know, I know, another account! But, it's worth it because you can then save your projects. You can use their service to create a variety of products such as calendars, cards and even canvas prints.
I like to just make regular photo prints of my paintings in 4 x 6 size with a matte finish and then using archival tape I affix it to the card of my choice which, I buy in bulk from an art supplier like Dick Blick. I like to do it this way so the buyer can choose to remove the photo and frame it just like a mini piece of art. All your uploaded photos are saved, making it super easy to do reprints.
The other reason I love using Costco is that they are fast and I can go in to pick up my order. Plus, they are super affordable. Each print is only 17 cents and when you are selling low priced items, like greeting cards, cost is super important. Disclaimer, my husband works for Costco so, I get free membership. I know not everyone has access to Costco but, maybe this will spark a resource you never thought of. Lots of artist use Vista Print or online printing services that seem affordable but, after shipping and minimum quantities, it can add up. Maybe look into your local printing shop or even Kinko's. They might offer fast turn around and quality prints at great prices.