Hi clearstream,
I've been going through a similar experience, i.e., trying to set up notecards on my website where customers can order them and I don't have to do any printing, stuffing, mailing, etc.
Unfortunately, no one wants a plain straight card with a print on one side. Not professional at all!
I did find out that MPIX does offer a greeting card that, when folded, is 4 x 51/2. The size is right and you have a print on the outside and can put a logo, or other information on the back. The print quality is top notch. The drawbacks are it is pricey, around a buck apiece for a minimum order of 25, the fact the minimum order is 25, and I'm a little concerned the type of paper used for the inside is a little slick for writing notes.
On the other hand, I wish Zenfolio would offer this MPIX product for us to use. It might have something to do with the software, which you have to download on MPIX's site to be able to design the card layout. On the other hand, if we as sellers do all of this and just offer the completed cards to sell, it should work.
In any event, the MPIX folded product is worlds above the straight card Zenfolio has given us to sell.