CBP Assigned Numbers
A Customs Border and protection assigned number is used when an importer does not have an EIN (Employer Identification Number) or SSN (Social Security Number). This is usually the first step a foreign entity would take to become a US Importer.
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What US Customs Says
The importer number (requested on Customs and Border Protection (CBP) entry paperwork) is usually the IRS business tax number assigned to businesses.
If you do not have an IRS business tax number, you may use your social security number in any paperwork requiring an importer number.
If you do not have either an IRS business tax number or a social security number, you may request that an importer number be assigned to you by filling out a CBP form 5106 and present it to the Port of Entry in your location where you will be filing your customs entries. The regulations governing the issuance of an importer number are in 19 CFR 24.5.