Yes, the same with USPS you have to abide by federal regulations. Your CBD has to contain <0.3%THC.

Yes you can, as long as CBD is legal in that country. If you're mailing smaller amounts you won't have to file a customs declaration either. If you're shipping CBD products that you've sold you have to fill out customs

Yes. USPS made CBD legal to mail as of June 6 2019 as long as it contains <0.3% THC.