Since it's not a food service job that pays the kid $2/hour, giving him a tip isn't a must-give-15% type of situation. Personally, I tip $5 as a minimum for good service.
On a side note, I wish the retail stores around me (SA and D***'s) had that kind of fast service. I went to one and they said it'd take 5-7 days to get my racquet back.
PS85, 367g/345sw, Pacific classic@58