V6.1 - Misc. Charges on SO - Per Piece?

Does anyone know of or have a good method for adding "Miscellaneous
Charges" to a Sales Order Line so that the charge is applied *per piece*
instead of per shipment/release?

Our current method is to add a second ("miscellaneous") line to the SO
(with quantities matching the first line) but the problem with this
solution is that it's very easy to forget to "ship" the second line
along with the first, since the lines are not tied together. Perhaps
it's possible to link the two lines so that one cannot be shipped
without the other?

We often make partial shipments of the line/release, and invoice
accordingly, but we have a series of orders where the "miscellaneous"
charge line-item was not included in the shipment and therefore not
included on the invoice.

Thanks in advance!