@Chrispy Thanks for the explanation, if individuals are required to fill customs declaration I understand that will be a deal-breaker for many sellers.
Personally I received many USPS shipments before COVID from businesses and never had to pay customs, I was also shipping and never filled a declaration at least to the US and EU, Morocco have lots of free-trade deals no idea if it could be the reason.
USPS is important to people outside the US as well, being under attacks and budget cuts we might never be able to buy from the US again.