If Frito-Lay gives this kid anything, they're acknowledging even the possibility that it may be true that they let a mouse in a bag of chips. The weight thing is an excellent point; these bags of chips are filled, packaged and shipped by weight.
Just to show how urban legends transcend facts, here is a list of food contamination stories you may have heard:
-Semen from HIV+ men found in food/restaurnts/pizza deliveries/ketchup bottles
-"Mayonnaise" on chicken sandwich actually pus from tumor
-Girl's salivary glands become infested with roach eggs after consuming contaminated taco
-Human finger found in Wendy's chili
-Urine found in steak after consumer falls ill
-Chicken head found in box of McNuggets
Now, here's what they all have in common... they're all BS except the chicken head which may be true because McDonald's is filthy and it's still under investigation (the woman will pose for photos with the head but isn't letting a professional examine it....).
In any case, the "I found a ______ in my ______!" stories almost always turn out to be false. I don't even think the kid is after money, I just think he was making a prank to gross out his friends.