Currently Canada is prioritizing visa for residents and citizens. All others are not eligible for dropbox too. Until you don't do landing, you are not a PR. Visa interview or dropbox is for people outside US. H1b extension is for people inside US. So you cannot send docs from US to Canada, you should be present in the country where visainterview/dropbox happens.
Do the softlanding, then book for appointment as PR status, you will get dropbox (other dropbox conditions also need to meet). Travel to Canada, stay there, do the dropbox and comeback with stamped visa.
Do the softlanding, then book for appointment as PR status, you will get dropbox (other dropbox conditions also need to meet). Travel to Canada, stay there, do the dropbox and comeback with stamped visa.