The refunds relate to levies charged by US President Donald Trump under the International Emergency Economic Powers Act (IEEPA).
“All importers of record whose entries were subject to IEEPA duties are entitled to the benefit” from the high court’s ruling, Judge Richard Eaton wrote in March.
As of early April, more than 56,000 importers had completed the necessary steps to apply for refunds online when the portal opened, with their claims worth $127bn (£943m).
The portal, known as the Consolidated Administration and Processing of Entries (Cape), went live on Monday.
A Customs and Border Protection spokesperson said Cape had been built to “efficiently process refunds”.
The system will process refunds for importers as one lump sum, rather than making businesses apply item by item.
Customs and Border Protection said successful applicants can expect refunds, as well as any applicable interest, to be paid in 60 to 90 days.
