How long will it take a postcard to reach the U.S from panama.?

The mail service between Panama and other countries is not that fast. It will take around 5-10 days for a postcard to reach the United States. More distant destinations will take longer. You can get stamps at post offices or from some nicer hotels. Most post offices are open from Monday–Friday from 7:00 a.m. – 6:00 p.m. and on Saturdays from 7:00 a.m. – 5:00 p.m.