To do this just add the Sales Transaction Entry window to Visual Basic. Then add the Quantity Fulfilled field and Drop Ship checkbox to your project. Copy the code below and paste it behind the SalesTransactionEntryDetail(Grid):
Private Sub Grid_AfterLineGotFocus()
Private Sub QtyFulfilled_AfterUserChanged()
Private Sub DropShipLock()
If QtyFulfilled.Value <> 0 Then
Dropship.Enabled = False
Dropship.Enabled = True
This will enable or disable the Drop Ship checkbox based on whether the Quantity Fulfilled is equal to 0 on both the Grid After Line Got focus and Quantity Fulfilled After User Changed events.
You can do this yourself as I describe above or download and import the package file with this code here.