At the shipping center, paperwork is filled declaring the amount of dry ice in the box and to identify it for special handling. Determine the type of shipping that will be used. By calling ahead, the shipping center will be able to advise as to what the best time of day for drop of is to make sure the package ships efficiently.
Please note that not all shipping centers are able to accept a package including dry ice, so be sure that you verify this ahead of time. The goal is to have the milk in transit as soon and as efficiently as possible. Having the breastmilk package sitting in a dock for hours only increases the risk for mishandling.