The only way is to use groups.

You actually can use groups no matter how many detail rows you have.

Though you can only set them up with 20 items of detail, once used on an invoice you can insert as many rows as is needed within the group, and they won't print if the group is set up that way.

To enter data from scratch that won't print, set up a completely empty group, pick it on a new invoice, then insert as many rows as you want (use the Ctrl-Insert key combination), then complete your data entry for whatever you're selling.