US Customs will send your firm a notice of non-payment and a fine. Your fine will increase + interest on your duty owed till US Customs will receive full payment. You, the importer will be put on a sanction list by US Customs and all future duty payments will have to be done by certified check. Eventually US Customs will collect the duty from the Surety Company that issued your bond. The Surety Company will use all means necessary to collect all fees incurred plus interest from the importer.