Please note: this is what has worked for me, and each person might have a different approach and they might ask for more information, but this is what worked for me and what you want to make sure you have sorted in advance. For Canadian Amazon sellers buying inventory in the US, or any business for that matter, sometimes it is nice to get access to US-based cards and also the great perks that come with it.
This is my referral link for the US Dollar American Express Platinum Card that I went with. This is a personal card.
http://refer.amex.us/ROBErCzqxW?XLINK=MYCP
Some people who are in the churning business will suggest getting a simple card and churning others later, but I wanted to get a great card that I will have for a long time. I also don’t plan on getting any social security number in the US or ITIN.
Some things I had in place before I hit the application.
✅I had a US-based bank account in my name and a US address. My address isn’t a PO Box and I think that is important that you make sure it’s not a known PO box type of place. Check using https://tools.usps.com/go/ZipLookupAction!input.action and put in the address you are trying to use, and make sure that “Commercial Mail Receiving Agency” isn’t marked as “Y”. Remember this is where the cards will get sent, so not sure I’d use your US prep center unless you’re super cool with them 
✅I had a bank statement ready in pdf that had the same address as the shipping I’m going to get my cards sent too.
✅I had a US based phone number. I use google Fi, https://g.co/fi/r/UYXA00 you can set up easily and turn off for 3 months at a time. It is a pretty cheap Sim card and allows for cheap roaming at the same data prices per Gig and not prepaid, only pay what you use. Not sure I needed this but I did end up verifying for my US amex app with my phone using it. Somes in super handy for traveling and keeps the same number unlike a prepaid sim.
✅Copy of my passport. I applied without a Social Security Number so I needed to provide another form of ID.
✅US Based bank Account – I use RBC Georgia. You will need to have a US based bank to pay the US Based Credit card.
✅I already had a Canadian Based Amex. Both a personal and a Business Platinum. https://jo.my/corpcard
Steps I took:
✅Click the link. Complete all this info with everything above and have it all ready to go. http://refer.amex.us/ROBErCzqxW?XLINK=MYCP
✅Check the box that says “I don’t have a credit history in the US”
✅Completed the application.
✅I didn’t get immediate approval. Asked me to call in and I called in right away with the reference number on the screen. Strike while the iron is hot! Don’t let it sit.
✅The agent asked to verify my info. I said I didn’t have a social security number and applied checking that box. He then asked for more ID. I had to give him my passport number and details on it. Just had to read it to him. Maybe the black box approval looks it up, so my passport is probably the most widely used vs say a driver’s license.
✅I also said I had a Canadian Amex but he didn’t seem to care. I was just trying to be happy and bro out a bit with the guy.
✅Not sure what back end checks and validations they were doing, but having all these items in place I felt ready to respond and had things all matched up with the addresses. I wasn’t going to get off the phone with the rep until he had whatever info he needed.
And that is how I got approved.
I will put a link here to another good discussion that happened in the past. I just finally decided to take my own action and get one.
Best of luck!