Ship My Gun has been great for me with NO issues with UPS.
When it is packaged and ready to ship, you take it to a central UPS outlet. This is not CVS, Walgreens, UPS drop off center, local UPS driver in your neighborhood, etc. This is the business where the big brown truck work out of. If you have it packaged up properly, with the correct shipping label the Ship My Gun furnishes, there will be no problems.
When I shipped out a rifle I sold last month, inside the box are 2 envelopes. One envelope has all the information that the receiving FFL needs on me and the customer purchasing the firearm. The 2nd envelope has an Apple Air Tag. The envelope has instructions for the FFL to drop it in the US mail drop box. The air tag is then returned to me.
For whatever reasons with UPS, your tracking number is not always updated regularly. I've had some shipments to me and from me that show a shipment was prepared and later delivered. It never showed it's tracking history.
Using an air tag with a self address envelope, you can see where ever it is at any time. I disconnect the little noise maker so it's always silent.