If you use our Tubes packaging types, you don't have to ship in literal tubes (you can use whatever packaging you wish to actually ship the items), but items configured to ship in smaller Tubes can take up unused capacity in larger Tubes configured for other products in the same order -- e.g., if Product A ships in a Small Tube, and Product B ships in a Large Tube with a Capacity of 2, then an order for 1 unit each of A and B would ship together in just 1 Large Tube, as 1 unit of A can occupy the vacancy in B's Large Tube.
Aside from Tubes, items configured to ship in the same Box or Envelope package type can share capacity in the same package together -- e.g., if Product A ships up to 4 units in a Regular Box and Product B ships up to 2 units in a Regular Box, then an order for 5 units of A and 1 unit of B would calculate shipping for just 2 Regular Boxes, since the vacancy in B's box could be taken by 1 unit of A.