Costco.com and American Express jointly run a promotion which lets non-members shop at Costco.com without incurring the usual 5% surcharge. To participate, go to Costco.com and enter 111775758538 member id at My Account page before registering (you will be prompted to register after you add an item to your shopping cart).
The offer is valid online only at Costco.com (will not work in store) and you have to pay with an American Express card. The promotion expires August 26, 2007. Click on the image below for more details.
Source: discussion thread at FatWallet forums (via WiredDeals)