moral dilemma. overchanged, return it?

[longhair]

i bought lunch at a bakery, it cost $6.70. i paid for it with a $10, the cashier must've thought i paid with a $20 and gave me $13.30 change.

i walked out without saying anything and i immediately felt guilty. it isn't too late for me to go back. should i or is it their mistake which i just happen to benefit from?