You don't. You ship it and see what the bill is when it hits the border.
It's part of why companies are halting shipments. You can try to guess what the upcharge(s) will be at the border(s) and pass that to the customer; but if you get it wrong (or it changes mid-transit), you're either charging your customers excessively, driving them off, or you're eating additional costs yourself.
TBH, I don't know. It would depend on your specific situation; I doubt we're just broadly accepting Americans.
Some resources:
Seeking Asylum in Canada
Paths to Immigration
Working Visas (>6mo, extended up to 4years)