from an anonymous source

I gathered considerable experience in mail-order fulfillment before going to work for the US federal government for 19 years. I believe it is a big mistake for foreign companies to ship merchandise into the US through private couriers rather than the US Postal Service. These reasons have less to do with service and reliability than with legal ramifications. Please consider the following points:

Nearly all shipments that enter the US fall into one of 4 legal categories:

1. Diplomatic correspondence and international transit mail, which may never be searched, seized or detained.
2. Sealed mail (airmail, registered mail, Express Mail, First-Class and Priority Mail), which may be detained or seized under strict regulations which include reasonable cause to suspect the package and generally require the issuance of a search warrant. Sealed mail need not be physically sealed against inspection, as was true many years ago. Sealed mail first passes through US Customs, where it may be passed, detained or seized either directly by Customs, or turned over to another US agency such as the DEA, FDA or Postal Inspection Service. To initiate such an action, the government must have cause, meaning the package must fit a "suspicious package profile", alert a dog or electronic scanning device, bear the return address of an embargoed company, or otherwise provide a tangible reason to suspect the package might contain contraband or merchandise for which duty has not been paid.
3. Unsealed mail (second-, third- and fourth-class mail, international parcel post and AO articles), which may not be arbitrarily inspected, but may be detained or seized in accordance with postal regulations less stringent than those required for sealed mail.
4. Cargo (all merchandise and goods shipped by private couriers such as United Parcel Service, Federal Express, Emory and others), which is subject to random inspection by US Customs officials at any time.

According to US Supreme Court decision, the protected status of the mails extends to the "mail cover" (i.e., the information on the outside of an envelope or package). None of these protections apply to cargo. Further details may be found in Chapter 7 of the USPS "International Mail Manual" and sections 115 and 124 of the "Domestic Mail Manual."

Personally, I would rather endure the problems and delays of using the US Postal Service in exchange for the far greater privacy and protection it offers.