Why wait for several confirmations before shipping?
After one confirmation, the transaction is very unlikely to be double spent. However, each further confirmation reduces the probability exponentially. Basically, the number of confirmations provides additional security that the transaction will not be reversed. 

One reason a retail store or individual might wait for more confirmations to ship is time; it only takes an hour for 6 confirmations to go through, which is negligible time for shipping a product, but can provide assurance to the seller.