Doesn't PMK recalculate the shipping after the drop ends? They have to adjust pricing based on the final order quantity (tiers) anyway, so you're getting a new (different) invoice in the end regardless. Much easier to tack on the actual shipping amount to that. I think what they've done in the GB is put a base shipping price on each parent & child deal so that if people opt to buy only 1 kit (like just the irradiated vaults, for instance) they still get a minimum shipping price on their order so they don't get a shockingly higher invoice after it closes.
Basically what I'm saying is that the money PMK reserves when you place your order is rarely what you're actually invoiced/billed for after the GB closes.