Goods received in error. Am I obligated to pay?

You placed an order with the company. Although you failed to pay them in advance they were good enough to send your order anyway, so now you certainly do owe them the money. Or, under the Distance Selling Regukations, you have SEVEN days to send it back to them if you don't want it. So, as Douglas advises, pay up or send back - or expect big trouble!