Related Clues:

• Overexert
• Return topic
• 15 percent, often
• Invoice add-on
• You can bet on it
• Diner's prerogative
• "Cogito, ergo __"
• It's left behind in good restaurants
• Revenue source
• Trackside advice
• Final answer?
• Grand total
• Price percentage
• It's an imposition
• Object to find in Kakuro