...for my invoice section I don't use line items (as you can only add one record from the main inventory to one invoice).
This isn't really correct. The line items table (usually a portal on the invoice layout) is used just for this purpose. Then you would print from a layout based on the line items table
You may be better off quickly jumping out of list view printing the current set and them returning to list view if you need to. Printing in FM has its challenges.
Although it sounds like you might want to rethink how your line items are managed all together.