UPS requires so many customs forms that one could easily conclude that they don't really want to be bothered. The USPS wins hands down on ease of international shipments. Prices vary so you may wish to shop around. If you are a frequent shipper, you may wish to consider creating an automated invoicing system. With a dot matrix printer and a little experimentation at form feeding, you can print out shipping forms without using a pen.
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There are pros and cons with both but the choice usually boils down to what is convenient for the shipper. If you use package delivery stores, however, the rates are substantially increased as most add a surcharge to the standard UPS rates. Before using package stores, do some serious price comparisons if you have any other options. Delivery time may vary greatly but with UPS your package's delivery status can be traced to some degree or other depending upon which shipping option you choose. Since the first piece is always the most expensive, one multiple piece order saves lots of money over several single piece orders.