It is possible to do custom declarations and postage online through USPS.com. Then give it to your carrier or drop it in a mailbox.
No driving to the P.O. or waiting in line involved.
Shipping by Registered Mail has to be done in person.
You cannot, however buy insurance on international items online. And you need the special clear plastic envolope for the customs form. You can order the envolopes from the usps. But I can never seem to find them when I need them.
Go not to the elves for counsel, for they will say both yes and no.