You'll need to do a couple of things:
1. You don't mention whether you're running an individual invoice based on an invoice number filter or multiple invoices. In either case, you need to group on Invoice Number to get this to work.
2. Create a running total that counts the number of rows as the report is running. It should count something that is unique in each record and it should reset on the change of Invoice Number. I'll call this {#RowCount}
3. Create a blank Invoice Group Footer section for rows 2 through 20 (or however many rows you want to display) along with a final group footer that will be used to close the box around your table (this will be the BOTTOM section!)
4. In the section expert for each of these blank sections except the last one, make sure that Suppress if Blank is turned OFF. Then click on the formula button to the right of the Suppress checkbox (DO NOT check Suppress!) and enter the following:
{#RowCount} > x
where x ranges from 2 to 20. This should give you a blank section for each row up to 20.
-Dell