Love the ShotPut crew. Some of the hardest working founders I've met.
Admittedly we were damn pretty terrible on the shipping and fulfillment side of our business early on and they've been a huge help getting everything figured out for us.
Working with Pakible is phenomenal we've recommended all our customers who want custom boxes to use their service! And we already handle their fulfillment.
Generally we call inserts: marketing messages companies put in each shipment.
There's another material called dunnage i.e. fill that is placed such as air packs, bubble wrap, paper, cardboard, mushroom (yes a product made from mushrooms!), etc. are used to keep the product tight.
Another method is to use a smaller box which cuts down on both shipping costs as well as is a more secure way of securing products.
Admittedly we were damn pretty terrible on the shipping and fulfillment side of our business early on and they've been a huge help getting everything figured out for us.
You definitely want to work with these guys.