Debit cards work differently than credit cards.
When you use any credit or debit card, the first step is a pre-authorization step. This ensures the merchant that they can actually bill the card for this authorized amount.
The merchant can only capture the funds when they ship the product. This is the law.
For a credit card, this pre-authorization is good for 30 days. This decreases your available credit, since you have told the merchant you intend to purchase. If the merchant has not captured funds before the pre-auth expires, the available credit goes back up and the transaction is voided.
For a debit card, the bank reduces your balance for the pre-auth period. The period is shorter for a debit card, but the process works the same way. Three to five days after the pre-auth expires, the bank will return your funds to your account.
Only on shipping may the merchant actually capture the funds.
What happens when the merchant cannot ship before this pre-authorization expires is that they need to start over, charge the card again with both a pre-auth and capture step (combined into one transaction). In the case of a credit card, there am not be enough available credit and your purchase will fail, causing a delay in shipping. In the case of a debit card, if the merchant references the original transaction within 30 days of the pre-auth expiring, the charges will go thru regardless and if there are no funds in the account, you will be over drawn, with all the fees and penalties that go along with that.