I went to a restaurant in Boston last night and the bill (incl. drinks) was $39 ($37 before tax). I paid by credit card and wrote down $45 in the total section. I now found out that $47 has been debited from my credit card from this restaurant. I was under the impression that 15% tip is standard, 20% tip for exceptional service (based on pre-tax amount). Am I wrong? Can the restaurant change the credit card slip after you leave the restaurant?

This is a neighbourhood restaurant (certainly not fine dining) and the party consists of 2 adults and 1 toddler (13 months old who did not eat nor require a highchair).