Repricing freight charges based on actual shipment date

Hi,

We are on 12.1.3 and have a requirement to fetch the applicable freight rate based on the actual shipped date of sales order line. The freight modifiers are setup to have different rates for each month. For example, the start and end date on the first line on modifier is 01-Aug-2012 and 31-Aug-2012 and the lumpsum value is 100$. The second line on the same modifier has start and end dates as 01-SEP-2012 and 30-SEP-2012 and the lumpsum value is 105$.

When the order is initially created (not copied) on 3-Sep-2012, the freight charge of $105 is automatically applied, which is fine. However, if the order line gets shipped (backdated) on 30-Aug-2012, then the reprice activity should change the freight to $100 (not $105).