Is the seller entitled to impose additional customs fees and taxes on the buyer, and to renege on providing an agreed-upon free sample, after the buyer stipulated "door-to-door" delivery of the goods without additional charges?

First, if customs fees were recently imposed and the seller was not aware of them, they are not binding on him. However, if they were imposed before the contract by a period long enough for him to likely know about them, then they are binding on him.

Second, the free sample can be one of two things: 1. A gift from the seller: The majority of scholars permit the seller to revoke it before the buyer takes possession of it, and there is no sin upon him. A gift becomes binding upon possession with the permission of the giver. 2. Part of the merchandise: This is the more apparent interpretation, and accordingly, the seller must give it to the buyer. The paid price includes both the merchandise and the gift. If the seller refuses to send it, its value may be deducted from the price.

The closest derivation to what is correct is that it is a pure gift, especially if the promotional gift is not of the same type as the merchandise. If it is of the same type, then it is considered an increase in the merchandise. If it is said to be a gift, then you have no right to deduct its value from the price. If it is said to be part of the merchandise, then if the seller refuses to send it, its value may be deducted.