You should add a "message" or "label" to the "Click to Pay" URL. Perhaps something like "Bitcoinstore.com order# 18272542". Then you are autodocumenting the transactions for the users, and you'll appreciate them having easy access to the order number when they contact you for various reasons.
Technically it should go in the "message" field, but I think Bitcoin-Qt doesn't really use that field, so I think it has to go under "label." I've talked to them about making the "message" field useful (marking the transaction, instead of the address), but they don't do it, yet.
This sounds like more of a comment for
Bitpay.com since the checkout is handled by them.
I don't see it as much of a problem currently since all the information is emailed to the customer as soon as they place their order.
The issue is not that the user
needs the information in their Bitcoin client, but they can't just leave the label blank or else they'll never know what any of their addresses or transactions were used for. Some are too lazy to type in their own description, and if they don't, they'll have fun trying to find old emails at a later time to cross-reference against their BTC transaction history.
Most importantly, when the merchant does it, they get a nice uniform labeling that clusters all their purchases together when they sort their tx history by label.
On top of that, it's stupid simple to implement, and it makes shopping with BTC literally a few clicks (plus maybe an encryption passphrase). I'd hate to see something so simple and useful go unused.
I can email BitPay about it, but I'm not a customer of theirs. I figured I'd persuade you to see the value and request it yourself