Each vendor is different, but Sweetwater will date your invoice for when you place your order, as a courtesy in situations like these (which they are well aware of).
Also, they have a three-payment-plan option, which might help you accelerate the purchase.
I unfortunately have to skip this wonderful offer, as I'm going to be out of the country for a month and have to pay two months of bills in advance, while not having access to any sublet money or other non-auto-deposit sources of income. It's a bummer, but there's also the individual download library offer, which I plan to take advantage of.