Always post to the shipping address that the buyer selects during checkout.

If the buyer uses PayPal this address will be on your PayPal payment received notification.

If the buyer opts to pay by Bank Deposit there's no longer an "I'm sending payment" email so you need to check the View Order details to get the shipping address.

[edit] And if you use C&S, it picks up the address from eBay checkout so you'll never have a problem with using the wrong address.