A couple of corrections: you are not registering your gun with Customs, you are registering (documenting) that you are in possession of the firearm prior to leaving the U.S. You can then return to the U.S. with that documentation that shows you owned the firearm prior to your trip overseas. Customs does not keep any record of your paperwork. It is a good idea to also have cameras, binoculars, etc. on the same forms and present for the inspecting Customs officer.
I did not make an appt. for Customs inspection (I live in a large city) and signature of my paperwork, I just arrived at their office with the completed paperwork, went inside (leaving the firearm secured in my vehicle, unloaded), got a Customs officer who came out to my truck and inspected the firearm. He signed the form, wished me good hunting, and I was off.
As for SAP paperwork and registering your firearm: I used a service and it was worth the price of $100. The gun service folks told me I could do it myself but if I ended up in line behind six guys from Spain who can't speak English and there isn't a SAP officer who speaks Spanish, I could end up in line for quite a while. After that long flight the last thing I want to do is screw around in a line for paperwork issues.
The gun service folks also won't put up with the bribe stuff that I've seen in the J-burg SAP office.