I know I have used the $20-$500 variable load Amex at Costco with no issues. I think I have used the $100 fixed also.
Costco registers are also smart enough to use the full remaining balance if your bill is higher.
Related question - has anyone used a $100 Visa or MC gift card with a PIN set as a debit card at Costco? I prefer those for places that don't take Amex, but would like to drain them at Costco too. I know I could buy Costco gift cards at costco.com with them, but I don't plan that well.