Out of Stock

After getting an "e-coupon" from CVS for $10 off $50 with free shipping, I filled my e-basket and then tried to checkout. Upon "Continue checkout", they informed me that two items were out of stock and so my order did not qualify for free shipping. So it took me another 5 minutes to find other products I needed. Grrr. That's not up to CVS's level of service. Why does their commerce software allow you to place items in your cart that are not in stock?