We are heading out on a 3 month road trip. We have a neighbor girl picking up our mail and just keeping an eye on the house.. no pets, no moving in, no lawn care. What is a reasonable amount to pay her?

Some neighbors would do this just for neighborliness. A nice gift from your trip? It doesn't "seem" to be like house sitting but it IS a LOT of service to you.
There is no "reasonable" amount here. You know your neighbor's circumstances and if she would like to be paid.
You say "girl" which may mean someone younger? (I sincerely hope so). then maybe a dollar or two a day which for a young person would be a nice payday.

Of course you must pay her. How much will depend on her age but, if unpaid, you know how it goes, you get what you pay for. Payment implies respect and you want someone who's keeping an eye on possibly your biggest investment to feel respected and motivated to do a good job and hoping to be asked again.

If she is just a girl and under 18 I would pay her as Gretchen mentioned. If she is in fact an older woman then I would ask her and depending on what she says I would either pay her or give her a gift card for her favorite local restaurant or theatre. Or ask her if you can return the favor.

Go to the post office and have delivery stopped. Tip your landscaper.

If I remember correctly, the PO will only hold the mail for 30 days. Not sure I'd want all the minimum wage landscapers in town to know the house was empty.

How old is she?

She would have to come to your house every single day except Sunday for THREE months. That could be big imposition depending on what's going on in her life . . . So I certainly would pay her more than $1 a day. Maybe $50-$60 per month so $150-$200 for the full three months.

I am assuming a teen ager -- right?

If she is very young - that may be too much money -- but also too much responsibility for 3 months.

Thank you all. These were very helpful responses!