Help with shipping question? UPDATED

Why do your parents have a PO box? Do they live in a town that does not have residential delivery otherwise? Or did they get a PO box because it's more convenient or something?

Is their PO Box the same zip code as her home address? If yes, she'll probably get it anyway, especially if it is a small town. May want to call post office to where it was sent to explain and ask for suggestions.

I have same problem sometimes with PO Box I have because rural and local post office is less than a mile but no delivery. If I got residential (street) delivery it would mean a 10-15 mile one way trip anytime I'd have to go pick something up that needs signature etc. Different town and zip.

Unfortunately, when you order something you may be told it will be delivered UPS or FedEx but then it is in turn dropped shipped for USPS to do final delivery. Our post office and UPS are aware of the mess so I always address everything to my PO Box # preceded by street address and with USPS town/zip. My PO is ok with it as UPS is too because they know of the mess carrier partnerships/drop ships has created. They make it work.

ETA  and I worked for the USPS MANY years and I was in on a meeting when they said USPS would be going into this work share programs with UPS and FedEx. UPS/FedEx didn't want to deliver to the way out in the middle of nowhere addresses and USPS had to anyway so perfect, right?  I remember mentioning it wouldn't work in many rural areas and why. (different zips for PO Box/residential).  They said  the kinks would be worked out soon. I can't even remember if I had kids then and eldest is in college now.... 
I think they should just simply have different address lines for USPS vs UPS/FedEx to the door deliveries.  But maybe I'm oversimplifying things.  What do I know?  

Ha. When my parents got tired of our box being taken out (we lived out in the country and I am not sure if it was being taken out by careless drivers in the pitch black, snow plows, pranksters, or a strong gust of wind), my dad's friend welded a box and post from iron and they put it in with concrete.  That thing was in there SOLID.  The mailbox would always win after that.

In light of your answer, my guess was going to be that if they live where there is otherwise delivery (ie, as long as it's not a town where everyone has to have a PO box), they would stand a good chance of getting it.  But if they don't have a box out front, then that complicates things.  It is also possible that their local PO knows them well enough to re-route it to the PO box instead of taking it to their house.  

Otherwise, I feel like I might call my mom and fess up and let her know what happened.  She might say "oh, we occasionally get mail at the house" or "Oh, I'll go see Bob at the post office and have him keep an eye out" or something.  Or call dad if he's more likely to be the one who knows the PO folk.

Another choice would be to just keep an eye on the tracking.  If it gets returned to sender, and you can afford to be out for the cost of two, then I might re-order the gift, and worry about dealing with the company for the refund on the missent one later.  

I don't feel very helpful, but I feel your frustration-- it's really hard to know what to do sometimes when companies refuse to specify what shipping company each option is going to be.

Hope your gift gets to your mom safely. 

They should get it.

My mom has a PO Box because there is no residential delivery in her town of 500 people. You'd be amazed at how often companies screw the address up in various ways. She always gets her mail in the end. Since the post office is so small if something just comes with the street address only by mistake the postal lady will find out which PO box it belongs to and put it there for you.

You're not being completely silly. I live in the country and we have both a PO Box and a street mailbox. I never know where I'm going to get mail -- things I specifically address to my PO Box get sent to the street mailbox and vice versa. I used to think it was completely automated but I'm pretty sure now, at least where I am, it goes to the zip code regardless of street address and they sort it further at the local Post Office. Good luck!