I don't think it's fine we're subsidizing domestic shipping rates to mail to foreign citizens on foreign soil, save maybe a few rare exceptions like they're acting in an official capacity on behalf of the US.

Are American citizens not entitled to send mail to other citizens overseas? I would argue that most mail that is sent abroad is between American citizens and their families (military care packages, Xmas cards for expats, etc).

Well, we could have a government policy of subsidising international mail sent from the US.

China does that, for example - which is why you can get a $3 widget on AliExpress sent from China with free shipping.

However, right now congress doesn't act like they're doing that intentionally. That doesn't seem to be the policy they're aiming for.