if your orders are being placed on Amazon.com and shipping out of the country then the buyer is responsible for all customs duties.
If you are selling on Amazon.mx and Amazon.ca and you are shipping from the USA then YOU are responsible for any customs duties. As well as having an address to accept returns in those countries. if you do not then you will also be responsible for all return label costs. Or fully refund without a return. Also Amazon will charge you an additional 3% currency conversion fee when it’s time to pay you from those countries.
So when we were on Amazon.ca we used DHL ecommerce.
They shipped the items from USA to the buyers in CA.
If during import the item value was above a certain $$ amount a custom fee would be applied. DHL paid those fees so the order would not get stuck in customs. Then they bill us weekly/monthly (depending on how your account was setup) for all the duties they paid.
Long story short…we now only sell on Amazon.com. The others don’t bring enough sales to the table to make it worth the trouble. You’re better off opening Amazon.com to international buyers if that’s what you feel you need to do.