It depends on which page size you pick in Step 1 of Product Labels:

• Product Packing Slip is available on several sizes, including 4' X 2', 4' X 2' Vertical, both 4' X 2' Packing Slip sizes and 4' X 2' Thermal — none of these require an extra module.
• Product Invoice only appears on the two large page sizes, 5.5' X 8.5' Packing slip (2 per Page) and 11' X 8.5' Packing slip (1 per Page), and both of those sizes also require the Packing Slip module to show up at all.

So if you don't see Product Invoice, switch to one of those two large sizes; if the module isn't active you won't see those sizes as an option in Step 1 at all. Once you pick a qualifying size, the matching Product Packing Slips (size) / Product Invoices (size) groups appear in Step 2.