Replace Blanks With Adjacent Cells

I use an old accounting program to download sales history into excel. For sales where there is only one line item and therefore 1 row, there is no problem. Sales with multiple items show a summary of the sale in the first row and the line item details in rows after.

On multiple item sales (shaded in yellow), the total postage is shown in the summary row (shaded in orange). The postage fields in the line items are blank (shown shaded in blue). I need to split the total postage by number if line items and put the value in the line items. i.e. if the total postage is $10 for two items, I need to show $5 for each line item.

The summary row identifies the number of line items in the "Qty" column ( cells in green). This cell could be used to divide into the postage (in orange) and also count how many rows below to put the answer in