Hi! @To-Youth
I don’t know what would be cheaper, because I haven’t tried to make postcards yet.
Here is a topic listing online shops for postcards, maybe you can compare the prices:
https://community.postcrossing.com/t/waar-kun-je-leuke-kaarten-kopen/7002/2
I don’t think all cities have their own postmark, there are only a few sorting centres in the country.
But collecting postcards from different Dutch cities sounds like fun! 