This is all fine and good until I have more than 1 customers who haven't paid. I somehow need to verify which customers have paid and which ones have not. But I cannot tell that by looking at the addresses of coins I receive via Bitcoin. Okay, maybe the items cost different amounts and I can distinguish the orders by looking at the price, but say all the items cost $5.99 so price alone can't determine the customer.
Is there a solution to this problem? Do I have to create a new address for every new order?
Interesting question. You could, as Neereus suggested, create a different bitcoin address for each transaction.
Another possibility would be to
embed a text message in the transaction. There's no provision in the GUI for it, but the source code allows for it (it's just not implemented at the moment).
I did not know that you can send messages with your transactions. Are these messages readable by participating bitcoin clients or something like blockexplorer (if they were to modify their website to include these fields)? Also; how about using this message field to include a GPG public key. That way if someone sends me a Bitcoin, they can also send me an encrypted message through a different messaging protocol which I can then decrypt.