Jan 21, 2018 Update: Stamps.com has launched a new International Flat service.  Get more info on how to ship merchandise with International Flats. Merchandise No Longer Allowed in First Class Mail International Flats If you ship merchandise abroad using USPS First Class Mail International Flats, there’s a new rule going into effect soon that you … Continue reading Important: USPS making changes to First-Class Mail International Flats