Question:
At least for a little while, that is.
Feeling pinched by high credit card merchant fees, 200 Shell gas station owners in Las Vegas, Little Rock, and Knoxville have decided (in cooperation with Shell) to do a test program where the stations will temporarily not accept American Express cards. (link) At these stations, Visa, MasterCard, and Shell credit cards (including the Shell/Citi MasterCard) will continue to be accepted (no word if Discover will still be accepted, though).
The reason? American Express typically has the highest merchant fees of the four major credit card networks in the US (2.5% average compared to 2% avg. for Visa/MasterCard and 1.8% avg. for Discover). But before deciding whether to drop AmEx nationally or not, Shell wants to see if sales will be affected.My thoughts: They might lose quite a bit in sales, as American Express dominates the corporate expense account market, and American Express consumer cardholders tend to be fiercely loyal. You can bet that American Express will fight to keep Shell like they did with Walgreens drugstores a few years ago over similar issues.

Answer:
Costco Wholesale Membership Warehouses (now Costco/Price in some areas) used to only accept Discover cards, checks or cash. For some years now they have only accepted American Express credit cards so I am sure they have negotiated a super rate, kept undisclosed to prevent just what the Shell dealerships are doing, from AMEX or they wouldn't have switched. Costco sales numbers are considerable.
American Express does have the highest merchant rates of the major cards and many businesses won't accept them for that reason (and a few other reasons). Most all who has an AMEX card have either or both MasterCard and Visa also so it really isn't a problem.
200 stations probably won't pose any significant impact on AMEX bottom line so I doubt if they will cave in and lower for them rates as then, when that word gets out, there could be a larger, general revolt against the higher rates by other merchant groups, and that could impact AMEX enough to make enough of a dent to force them to lower rates.