Why do you say there is such a long time gap?  Obviously it might be a while before it gets in a block, but you'll see the pending unconfirmed transaction very quickly.

If you don't think seeing the pending transaction is reliable, you could have a button where the user could click 'payment sent' which would mark the cart as 'payment pending' anyway, to prevent double-pays.  Obviously you would still wait for confirmations before sending the goods.

The way this "should" work in the future is that when the user goes to check out a cart, they get a payment protocol message with the payment details, which their wallet will hold for them until they decide to pay it.  That cart is immediately marked as 'payment pending' and you wouldn't accept further payments for those items.