Create an expense account named clearing, and then when the additional bills come in:
1. Pay the shipping/customs bill or enter the bill and use that clearing expense account as the expense for the transaction
2. Edit the original bill as follows: 2a. increase the total amount for each item by a portion of the new bill 2b. change to the account portion of the bill at the top, select the clearing expense account, enter the amount of the bill as a negative number 2c. save the edited bill and click though any warning you may get about payments being applied. 2d. Insure the clearing expense account is zero balance, if not go back and adjust the item totals.
QB will use the edited bill to update the purchase cost. If any of the items were sold between the original bill and the additional bills, QB will automatically make the adjustment to the COGS account for the change in purchase cost.