It's up to you. Ilford's postcard paper is the most common choice. I made cyanos once, on Stonehenge. It was alot of work, but was appreciated. FB papers show up, sometimes 30 year out-of-date ones. If they are double weight, they stand up to the mail just fine. Single wt might get creased. Some folks put their postcards in envelopes. Photomat colour prints, with a stick-on label have made their appearance too. If the post office will accept it, so will APUG.
In Canada it costs the same for a letter as a postcard, and that is going up by 1/3rd this spring - which is going to price me out, I think.
I don't think the rates go up until the end of March - maybe one more round?