How to enter shipping weight/dimensions when supplying from Alibaba (FBA)?

I’m at the point where I need to enter the weight and dimensions of my shipment into Amazon to ship for inventory, so they can know how many boxes to use. I asked my Alibaba supplier how much my order will weigh etc. and they said they don’t know until I actually place the order…

I’m ready to place the order, but I can’t actually place it until I have an address (Amazon warehouse) to give them. Amazon won’t give me an address until I set up this inventory info. It’s a circular problem.

If you can’t tell, I’m still a newbie, but it seems like I have everything figured out except how to get my product physically from Alibaba to an (American) Amazon warehouse. I’m sure there’s a simple solution, but I’m afraid to just guess or mess something up that could end up costing me hundreds of dollars, or getting my shipment lost.

Any advice is greatly appreciated. Thank you!