Using First Class Mail International, deliveries generally take between 7-21 days to arrive, although the USPS does not guarantee delivery dates or times.Mail may travel to its destination by ground, air or a combination of both.ou will also need to weigh your mailpiece to ensure that it does not exceed the weight limit for First-Class Mail International deliveries, and to calculate the cost of mailing. Stamps.com software will automatically perform the rate calculation based on package size, weight, and delivery destination.