In message , at 17:53:07 on Fri, 3 Mar 2017,
tim... remarked:

A transaction can be accepted in full, accepted at the retailer's
risk, or refused completely.

yeah

but what does At retailer's risk mean

It means that if the funds don't eventually flow from buyer to card
company, neither will they flow from card company to seller.

If you are selling something intangible like tube fares, it may well be
better to take the risk that the cardholder will put money into the
empty or maxed-out card account, rather than **** them off by refusing
to allow them to travel.
--
Roland Perry