If you have to post the thank you cards touchnote might work. We use touchnote all the time so we buy bulk credits that work out £1 each for a postcard photo to be sent anywhere in the world (we have family abroad, for us it is cheaper than a card and a stamp). Individual photo postcards are £1.50. www.touchnote.com/
We found vistaprint have lots of hidden charges and their free or standard delivery takes about 3 weeks! they bombard you with emails continually afterwards. Photobox are very good and v prompt delivery
Just that really, we're looking for an online service that will do cheap, simple photo thank you cards (that don't take 14 days to deliver or make you pay extra if you want to write on them!). Thanks in advance.