I just ran into the Form 5106 issue for the first time with FedEx. Have an inbound watch from Europe that got to Memphis last Friday (3/18) at midnight, but no FedEx customs people work over the weekend. This Monday morning a FedEx Customs Specialist sent a note to the sender saying they needed the watch breakdown (which was already presented to them) and a completed Form 5106 from the receiver (me). Sender sent another copy of the watch worksheet and asked FedEx to contact me about the Form 5106. The FedEx agent sent me an email with a copy of the Form 5106 and the instructions, and I was told to fill it in and email it back to him. It's a Department of Homeland Security Form and it is pretty simple; name, address, phone, email and your Social Security Number which acts as your Importer Identification Number within CBP (Customs and Border Protection) for future use. You can opt to have a CBP number assigned to you instead but you still must provide your SSN on the initial form. So if you don't provide your SSN up front you aren't getting the parcel.
I completed the form and emailed it back to the Customs Specialist and 30 minutes later I got an email from him saying the shipment has been released, and the FedEx system is showing delivery tomorrow.
The smoothness of this process is somewhat agent dependent, in this case it was all cleared up in a couple of hours, but the sender, the FedEx agent and I were on our toes to get things done quickly. But if one of those people was non-responsive due to travel or a wrong email address or something else the system will grind to a halt.
(Many people in the US are reluctant to share their Social Security Numbers but they have been so compromised over the past few years through hacks of the three credit reporting agencies that they are all over the dark web. I have my accounts frozen at the agencies so if someone has my number they can't open credit in my name because credit guarantors won't open accounts without being able to access your credit files). It's the result of living in a digital age.