My country has no postcard rate, and for a USA letter it is something like $1.05 now, and beyond USA, 1.85.
If you wonder why you always get a collection of 2-3 prints from me, it is because I want to get my money's worth out of these expensive stamp rates
For me that is any letter sized document to 30g.
Paper size isa personal thing. A paper cutter in the darkroom really opens up possibilites.