I think it would be better if you create a group on InvNo and put Subtotal in the group Footer.
To get the result below, you can preprocess data in a cursor, mark the last record in the group and use that mark in the Print When for SubTotal. ALternativelly, you can call following function in the Print When for SubTotal.

FUNCTION LasrRecInGroup
LOCAL lcNextInvNo
SKIP
lcNextInvNo = InvNo
SKIP -1
RETURN NOT (lcNextInvNo = InvNo)

>how can I get this kind of report:

>InvNo     Item Description    Qty    @Price      Subtotal
>---------------------------------------------------------
>10001     AAAAAA               10      100
>          BBBBBB               10      100
>          CCCCCC               10      100         3,000
>10002     AAAAAA                1      100
>          BBBBBB                1      100           200
>
>instead of:
>
>InvNo     Item Description    Qty    @Price      Subtotal
>---------------------------------------------------------
>10001     AAAAAA               10      100         1,000
>          BBBBBB               10      100         2,000
>          CCCCCC               10      100         3,000
>10002     AAAAAA                1      100           100
>          BBBBBB                1      100           200