Never use the US postal service to have anything guitar related shipped
You have better ways to ship and receive things in the mail, and I will tell you what they are.
It's common knowledge that the US postal service is in huge trouble. Shipping or receiving anything from them is just a bad idea.
In the part of the US where I live, the post office near me has a Google Reviews rating of 1.7 out of 5 at the time I write this. Yes, it's that bad.
I have a PO box there not because I want to but because I have to. Mail has been lost. The USPS "Informed Delivery" feature is almost worthless, as that system will state a package is there when half the time it's not and gets there 1 to 2 days later. If I receive a yellow card in my mailbox informing me a package was too large for the box and to go pick it up from the pickup desk, half the time it's not even there because of ridiculously poor sorting management on their part.
Again, yes, it's that bad.
Whenever I have to ship anything from there, I'm extra careful about it. If for example I sell something on eBay, I measure and weigh the box myself, then have the eBay system generate the shipping label for me. I do this because I can't trust that a US postal employee will enter anything in their system correctly, so I basically have to do their job for them. I purposely make it so easy that even a monkey could do it. All they have to do is put the box on the scale, scan it, give me my damned receipt (which I always insist upon because it's the only proof I have they accepted the package) and that's that since the label has already been paid for in advance.
How do I get guitars shipped?
I either buy a guitar from from Guitar Center and have them shipped directly to the store for pickup, or I have it shipped UPS. I don't dare have one shipped via USPS.
If I had to ship a guitar to someone else, I would only use a local UPS Store to do it. I put the guitar in a gig bag, drive over to the store and let them do everything. They box it, they ship it, done. Yes, it costs the most to ship this way. But it is the best way to ship a guitar.
How do I receive guitar related items?
If it's a smaller item (which it usually is), I use Amazon lockers. These things work fantastically well. The shipping charge for using a locker is usually free, and the best part is that I never have to go near a US post office. Buy the whatever-it-is from Amazon, it gets delivered to the locker, I get a code in email, I go there, punch in the code, take my package, then go home. Nice and easy.
The locker location near me is inside a 7-Eleven store and it's stupidly easy to use. And yes, lockers can be used for receiving or sending stuff back. Works very nicely.
For larger items that require me to use UPS, there's a specific process I will go through. As soon as I get the UPS tracking number, I call 1-800-PICK-UPS and request a package redirect either to the closest UPS distribution center or UPS store. They call this a hold for pickup.
For FedEx, they do the same thing and I will use it. I will call and have the package redirected to a local FedEx Office store for pickup.
Why don't I have packages simply shipped to where I live? To avoid delivery drivers that lie. I have had the experience where I've sat at the house all day waiting for a package only to have it never show up, call the carrier on the phone later to find out what the hell happened, then discover the driver just outright lied and said he showed up to deliver but never did. When I have packages redirected to hold locations, then they get delivered properly.
Yeah, I do quite a bit to ensure I get my packages. It does require me to go out of my way just to get my stuff. But I do get my stuff, and that's the whole point.
Amazon lockers are always your best option
The best reason to use Amazon lockers for buying anything guitar related can be summed up in one word: Accountability.
Amazon is both the seller and the shipper when you use their locker feature. They're accountable for 100% of the transaction when you buy stuff this way. From the moment you pay to the moment the thing you bought is in the locker, they're responsible and accountable for 100% of the fulfillment. Yes, this is a good thing.
The biggest complaint anyone has about the USPS is lack of accountability. USPS simply doesn't care at all whether you get your mail or not, and it shows. Amazon, on the other hand, does care. A lot. They want to keep you as a customer.
Believe me, you want a seller that actually gives a crap that you get your package. It matters. That's why you use Amazon lockers.