If the freight charges are not know until after the goods are received, you must wait until the vendors invoice is received before you can record the freight. In this case, the freight is entered in the detail of the voucher. The picture below shows a voucher entered with the total freight charge posted to the "Freight In", 6070, account.
The first detail line of the voucher records the total amount of the receiving slip, 62.00. The second line records the freight charge amount, 6.48.