Complementary items funcionality, or similar.

HI,

I'm searching for a functionality that allows me to add an item to a sales order, when a particular item is sold.

The scenario is the following: when vaccines are included in a sales order, we need to add ice packs. We tried using free goods functionality for the ice packs, but it didn't work because the relation between the quantities of the vaccines and the ice packs is not always the same, and sometimes we needed to cancel or change the quantity of the ice packs.

Basically, once the order is created we need to be able to modify both the vaccine and the ice pack lines individually, which couldn't be achived with free goods functionality.