Cautionary note:

Some merchants at PVG airport (Shanghai, China) offer DCC. The terminals are kinda compliant BOC terminals (will offer currency choice) but cashiers (at least at FamilyMart) are trained to choose foreign currency and the terminal has no "void" option once the transaction is authorized. I tried, fiddled around with the terminal for a good five or ten minutes trying to find it. It's obviously just hidden because the "function" menu is numbered and there are large gaps in the numbering. Attempting to type in a "missing" function number (I presume one of them would be "void") asks for a "bank password".

In that case the manager was called and after making a scene (and showing them that pushing "cancel" on currency selection screen did not cancel the whole transaction with an example purchase of a plastic bag) they gave me back the difference (and the charge for the bag) as cash out of the till. I thought they learned when the terminals started presenting currency choice again. Obviously not.