That's right gentlemen. It is at the San Pedro post office that the duty will be assessed and typically I have found them to be a little less expensive than the customs officers at the entry ports. However there are some shipments the post office can't receive - I don't know what they are but certain items that require permits will be held at the border to be cleared by a customs broker.

The initial question regarded a small plumbing fitting which I believe should be easily received at the San Pedro Post office.