This just happend to my parents last weekend, they found $150 on my little brother's wallet. They asked my little brother where he got the money , he said he just found around the house. My parents spent the $50 on groceries and gave the other $100 to the church during the mass.
If I honestly found $150 lying around anywhere, no doubt I'd take it, but hearing this just now makes me remeber how other people had a chance like this but they gave the money back and got back even more, more important than money.
But still if you found money in oyur mailbox somebody must have wanted you to accept that money??!?!?!!!