USPS has a lot of new restrictions when it comes to international mail delivery.

Apart from the country-specific restrictions and laws, USPS does not currently deliver all letters via the First-Class Mail International service at the moment. According to their support channels, for an item to qualify for First-Class International, it has to meet the following requirements:

  • Must weigh 4lb (64oz) or less
  • Package value must be under $400
  • Minimum height is 3.5"
  • Maximum length is 24"
  • Minimum length is 6"
  • Maximum combined length, height, and width is 36" (42" for rolls/tubes)

So, the main issue you might be facing is the height since letters/envelopes/flats do not meet the current height requirement. The package has to be a soft package or a box to meet the minimum requirements.

First-Class International shipping is available for packages and other options that meet the abovementioned requirements.

If you're attempting to forward a mail item to an international address using USPS and you get the error "First-Class International shipping is unavailable for this item. Please select a different delivery option or carrier." it means your item does not meet the current USPS requirements. 

In this case, you'll have the following options:

  1. Adjust the mail item details and choose a different delivery option from the predefined packages offered by USPS that meet the requirements.
  2. Use a different carrier or shipping option.
  3. Forward the item manually.

For more information regarding USPS First-Class International Shipping, please consult the USPS website: USPS: First-Class Mail International.