The way to to this is to use a group item, which is a type of container item that includes other items.  Group items can be set up so that their details don't print.

You can set them up with up to 20 items, and then when you pick them all of those items are added to the invoice, "wrapped" with the group item that works sort of like a subtotal.  You can then (optionally) insert more detail lines or delete unwanted ones.

To enter detail data from scratch that won't print, create an empty group item for this purpose.  Then pick it on a new invoice and insert as many rows as you want (use the Ctrl-Insert key combination), then complete your data entry for whatever you're selling.  And then none of that detail will print.