Mobile payment in China is prevalent. In fact, it is so prevalent that cash is longer trusted (in some places).
This sign, found in a shop window, tells its patrons that they need to provide proof; presumably identification documents; when paying by cash. Paying with WeChat – the Chinese version of whatsapp, AliPay and Unionpay are the recommended payment options.
Whilst there, I noted that many restaurants and shops gave patrons incentives to pay digitally. Vouchers, discounts and prize entries were frequently available if you paid with your mobile.