Shipping Container of Building Supplies to St. John

Hello everyone. I have been viewing this forum for a couple months and it has been very helpful! I have found other threads that are similar to what I'm going to ask but none of completely resolved the question I have. I have also talked to several customs brokers and they have come close, but are not willing to go into great detail with me because I'm still 6+ months out on moving the container. I'm planning on building a home in St. John and have purchased a lot. The home will be ICF and that material will ship from the states to St. John in a container. Since there is alot of free room in the container, I plan on stuffing it full with as many building materials as I can get in there. I understand I will be charged a fee on the materials (4%?) and that I will need a receipt of everything in the box. What I dont understand is the country of origin and the pricing structure. For example, I will have one box full of plumbing materials (fittings, valves, etc.) and that will all be documented on the invoice. I imagine most of these materials will have been made in china and have already had an import fee to the states assessed. I'm un-clear if I need to look up each item and say where it originally came from. That seems like a heavy lift. I'm hopeful that if the invoice shows the purchase was in the states then that is all that is required and no additional import fees will be assessed. Thank you in advance!