I always like to send out a postcard with an order. It looks nice in the box and leaves the customer with something they can post on a fridge or work area for next time. It also can let you highlight beads that normally aren't seen or considered for a piece of jewelry.
My husband (pre-retirement for 27 years) was an organic toymaker and we used postcards to reach his customers and let them know what his upcoming shows were. I can't tell you how many repeat customers we got off of those mailings.
This year I need postcards for the ISGB - IF I can get them photographed and done in time. It's always a time and energy problem, isn't it? UGH. Making the .jpg for the postcards is a relatively easy process and it's fun. Since most beadmakers and artists take their own photos most of us are familiar with editing programs. Translating that into a postcard is made easy by the many companies that make them.
I just finished my daughter's Wedding Invitations/ RSVP cards for this fall's festivities. She's a modern kind of girl but wanted something her friends and family could post of the fridge. We did a double card so we could adhere clear mailing stickers to the set and once opened they'd have all of the info they needed and a conveniently pre-stamped return postcard. Both cards carry a theme that is running through all of the wedding planning....she is a romantic, techno nerd, outdoor girl ;-).
Personally, I use Overnight Prints. But, I have used many others and they are good too. I'd play programs from several of the companies before you hit that "Buy Now" button. You can see what works best for you.
Other companies you might want to consider: 123 Print, Modern Postcard, Got Print, and Sharp Dots. There are others. I've only used 123 Print and Overnight. I hear the others are great but why mess with success.