Structures for a unitary spring system end cap include at least one rib formed on the outer wall of an end cap. The at least one rib can protrude from the outer wall, for example centered along the length of a spring system. Where a carton containing a product protected by a unitary spring system end cap is dropped from a height, the rapid deceleration caused by an impact with a surface causes the at least one rib to engage the spring system on which it is centered. A second such structure includes at least one bulbous feature located centrally of the platform of an end cap. The at least one bulbous feature can absorb compressive forces applied to the platform of the end cap, crushing in a controlled manner.