Like Joe, there are not any mailboxes (N. American type/style or any other type LOL!) in our area. I've never seen any anywhere either, but that's not to say there aren't any. Some folks have home made letter boxes on their front entry gate, but not many, certainly not us.
In all the time I've sent mail from overseas to here, over the last 15/20 years or so; or others have sent to us since we retired 10 years ago, the local post office have delivered to the house.
When we're not in, if it's regular mail it will be left with the sis-in-law next door. If registered then, again like Joe, they'll leave a card if there is no one in to sign. The postman has a PhilPost issued m/c and delivers to our Purok, as and when required. He's become a friend over the years and also has our mobile numbers!
When sending, ensure the full address is on the letter/card etc. etc.
viz.:House Number, Purok Number, Street Name (if applicable) Barangay, Municipality, Province and Post Code, Country.
If sending a small package/parcel, it will be helpful to write the recipient's phone number on the package, as they will probably have to go to the Post Office to collect it anyway. Otherwise, it may be waiting in the P.O. for some time, until a delivery to your area is scheduled.
It could help to write the recipient's phone number on regular mail, but you would have to ask her if she is happy with her number being in the "public domain".
When sending registered or EMS mail from here (similar to the Royal Mail in GB) a recipient's phone number is asked for on the PhilPost form. The requirement may, or may not, be on other countries' forms.
Footnote. It's called a "Post Code", not "Zip Code", so make sure you use the correct terminology.
Peter