If we wanted a truly contactless and secure way to do this we could also make use of a OTP (one time pin normally random 4 digits) when a waiter or barman wants to add a customer it sends an OTP to that customers cellphone. This will also be the easiest way to make it work on all platforms(Ipad, Android and browser) and will require no extra devices like card readers. This will also give the customer piece of mind that his / her account is secure.

Below is an example of what it think could possibly work.

Yes this is very important.