This video outlines how to get the USPS customer service number. Make sure to call USPS with the tracking number and full shipping address of any shipment you're contacting them about.
Call 1-800-275-8777 or the General Information number on USPS's website. You'll be greeted with an automatic voicemail, say "Misdelivery". You'll then get a list of options, to which the last one should be something like "More delivery + tracking options".
Repeat the last option for more tracking options and you should be redirected to a representative. You may have to wait on hold for a while (I've been on hold for up to 30 minutes, find other work to do while waiting with the phone on speaker).
Once you're in touch with a live representative, explain to them that you are the sender of a package whose recipient claims they have not received the package, so you'd like to open an investigation case. They may ask you for information on the business, which you are free to give. Make sure to give your name, your phone number, and the business address of 201 N Weston Lane, Austin, TX, 78733.
They will give you a Case ID (always begins with CA), which you must write down and keep on record, and they will ask for a number to call you on when they have solved the case. After that, email the customer letting them know that USPS is investigating, and then let them know again when USPS gets back with you which should be in 2-3 days.