In message , at 18:54:16 on Sun,
26 Feb 2012, Graham Murray remarked:
Uh, always? What do you do at a fuel station which requires a credit
card before dispensing fuel, or at a restaurant, when the tip amount
to be charged is not known until later, or at a hotel that requires
a credit card to create a reservation and the same one or another
one to check in?

At the restaurant, the card terminal could display the sub-total and
prompt for the diner to enter the amount of the tip (with the option of
just pressing enter to not leave a tip) before displaying the total
and prompting for the PIN.

Not "could", this is what they *do*.
--
Roland Perry