Special Charge not to split in Invoice

Hi Guys,

I have a situation here:

In standard order there are few line items. Some Materials (AMB MAT) are shipped at ambient temp. Some items are shipped in ice packs (ICE MAT) for which one fixed charge (Say Cond Z99) is applied (say $50 no matter how many items are there in the order which need ice packs, in sales order this $50 split among all ICE MAT).

Now there is split delivery taken place, now my user want Full Z99 charge should apply to the first invoice where in ICE MAT is there.

Help! Help!