Isis, the mobile commerce joint venture between AT&T Mobility, T-Mobile USA and Verizon Wireless, has announced a moving promotion for American Express card members. From now through June 1, 2014, Amex cardholders who tap to pay with a registered card through their Isis mobile wallet for a ride in a qualifying New York City Yellow Medallion taxi can get 50 percent back in statement credits.

The promotion is available to Amex's U.S. Consumer, OPEN Small Business and Serve cardholders who have registered their card in the Isis mobile wallet. The maximum allowable cash back through the offer is $100, Isis said.

In February 2014, Isis announced a promotion for Wells Fargo credit cardholders. Until April 30, consumers using a participating Wells Fargo credit card in the Isis mobile wallet will receive a 20-percent rebate in their statement on all tap-and-pay purchases, up to $100 per month, as well as a $20 credit the first time they tap to pay.

In addition, American Express Serve cardholders receive $1 back every time they tap to pay with the Isis mobile wallet.

The Isis mobile wallet is supported on more than 50 Isis-ready smartphones available on the AT&T, T-Mobile and Verizon Wireless networks. Customers with an Isis-ready smartphone need to get an enhanced SIM card from their mobile carrier and download the Isis mobile wallet for free from Google Play, for which messaging and data charges may apply, Isis said.