Is it permissible to sell halal products to a customer who is known to be smuggling part of them to another country without paying customs duties, given that it is not possible to refuse the sale?

You are not at fault for selling permissible goods in a lawful and legitimate manner. The responsibility for the buyer's evasion of customs duties lies solely with him, for a person is only accountable for his own actions and is not questioned about the actions of others, as Allah Almighty says: "And whoever earns sin, he earns it only against himself" (An-Nisa: 111). A Muslim is required to advise, enjoin good, and forbid evil to the best of his ability.