Really depends on the specific situation but I like to consider three factors with any freelance work: 1 - Who is it for and what could it lead to? If it's for a friend I would charge something different than if it was for a company. 2 - How long would it take (in this case for initial set up and per postcard)? You can then multiply this with an acceptable hourly rate. This should be based on question one but don't undersell yourself. You don't have to reveal this information to the client if you want to charge a flat rate per job. 3 - What's the pain vs. gain? Are you getting something that could potentially go in your portfolio and lead to more opportunities? If yes, would you charge less just so you can use it to your advantage? If no, is it really worth your time?