Customs duties and import taxes vary from country to country, and kick in at different values for different types of products.
If the package you've shipped contains products that qualify for that country's import taxes, your recipients will be responsible for paying these to their country's customs or tax department to receive the package.
Typically the process is the package is held by the country's customs department, and notification is given to the recipient by mail or phone, telling them how much they need to pay and how. Once the recipient pays whatever is due, the package is delivered. If the recipient doesn't pay the taxes due, the package should be returned to you, but it may depend on the country's policies.
There's no way to handle customs any differently when shipping internationally with USPS, and it should be expected by your customers if they've ever shopped internationally before. As long as you make sure to put some sort of notice in your checkout for international customers that they're responsible for any import taxes their country charges them, then the expectation should be set correctly and everything should be smooth 👍
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