handling paypal: 1 payment for multiple items that ship at dirrerent times

We have an item that can and will usually be ordered multiple times.
Payment is made up front for the lot.
We require signed contract to process the lot.
Shipping time is variable for each item.
Once an item is ready it is delivered to the customer online 1 by 1.
Each item is time sensitive so it must be shipped when it is available.

Because the payment was medium sized, Paypal is holding the entire funds until the entire lot is shipped...
Ideally, we should at least get the funds released for each item that was delivered instead of having to wait 21 days for the whole payment.

Options? Ideas?
Thanks!