That sounds about right Sign up for 30 prints, and you will have to send out 30 prints to your recipient list. You should get back approximately 30 prints (maybe minus a few) from those people. Every once in a while, a card gets lost or someone doesn't send out a card, so you might only get 29
Most people print on Ilford postcard paper (look for 4x6 Portfolio RC paper). Other paper is fine though. If you live in the US, and your picture is 4x6 or less, you can use the postcard postage rate as opposed to the letter rate.