Online shopping **** .. only once your payment has been processed do you get notified of items not in stock ... why not before?

Seems like online shopping is taking a similar trend, the orders are taken, then ONLY AFTER they payment has been processed do they inform you they don't have stock ... then they hold onto your money .... I was informed via email that the balance of my order would be delivered 6 May, it's now 11 May ... now try and track or call someone ... IMPOSSIBLE ...