Amazon’s process for international shipments is really not there. You will need to determine what you need to do on you own.

Normally, that will include shipping to a Canadian address where the shipment can be received and prepared for crossing the border. That means that before it gets past that point (and better earlier), you need to produce a commercial customs invoice that identifies everything in the shipment, quantities, country of origin, HS code, your Canadian exporting legal entity, your US importing legal entity, the number of boxes and/or pallets, the weights and dimensions of all. If pallets, you won’t need to be box weights and dimensions, but the weights and dimensions of the pallets. You should be able to identify everything that is in every box on every pallet and should be prepared to have Amazon labelling for every single box and to provide that detail to the customs broker, for customs clearance, if required.

The forwarder will ship across the border to a bonded warehouse while it clears US customs. Once cleared, either the same carrier or different carrier, who could be identified using the US Amazon partnered carrier process to pick up the shipment and deliver to the shipping plan destination(s) (could be several).

I have had shipments send to a logistics company with facilities on both sides of the border. They are able to take one or multiple shipments and do any preparation work you require, ship across the border, clear US customs, and either use the same or a different carrier (including the partnered carrier) to take the shipment(s) to the final destination.

If you do not pay someone to do all that, you will need to do it.