Well, I think what happens is that, even though they don't charge you until it ships, they still do put a hold for that amount on your card temporarily. That's what happened when I almost bought a MXP. They put a hold on my account for the cost of the phone for like 3 days, then it was removed. The customer service rep I talked to said that I wouldn't actually be charged until it shipped.
So, even if they are only putting a hold for that amount on your account, if you don't have the money to back it up, it will still get declined. That's my understanding, at least. My debit card was also declined when attempting to place the order.