I had my postcards done by overnightprints dot com and I am very pleased with them. The only thing that I will change next time is not to have rounded corners (optional extra), because when you upload the images it doesn't show how much the chop off!
I leave one stamped (for Europe) card in the gite with the option to buy more (also stamped - I hold international stamps, too) at 1€ each. This stops them from being used as shopping lists!
I tell the guests that they can leave them on the table in my porch and I will post them - easy in France because the postie will pick up from your letterbox