On a couple of occasions during a recent trip to the US, I paid in a restaurant when I ordered (at a walk-up counter), and was invited to add a tip to the CC slip. The food and drink is then brought to my table (waiter service).
What is the protocol for tipping in this situation? – I'm being asked for a gratuity before the service that I am to receive.
I accept the tipping culture in the US, but how do I gauge what percentage to give? Just the usual 15-20%? A bit less because they don't have to bring me the check? Or should I tip in cash at the end? Will I cause offence if I don't add a tip when I pay?