Not to take all of the fun out of tihs - but is there a need to match up the two children? As in - the order can have n shipments and m invoices - under what circumstance do you need to cross-reference a specific shipment to a specific invoice? As long as the shipment details add up to the total on the order and so do the invoices - what else is needed?

I just have a sneaking suspicion that the details might not exactly match, even if the totals tie in.

knowing the "why" might help with the "how".