this is a requirement from the US side, NOT CBSA (which is the canadian). To know for sure what the requirements are for the land border you intend to export it through, you will want to call THAT office. CBP will tell you what THEY need. Personally, the car being in the US for 72 hours didn't come up, then again, i dropped off the papers well before the 72 hour deadline. again, this is not a canadian RIV/CBSA requirement, it's a US/CBP requirement. Technically, the requirement for export is the papers and ITN# need to be submitted to the export office 72 hours before export, i'm not sure if it actually means the car needs to be in the states that whole time. Again, calling the export office at the US border you will be exporting it through will confirm this. remember, the export and import are 2 different processes, and deal with different agencies. If you show up at the US side and say that CBSA said it's ok, they will probably say something like, "that's great. we're CBP and WE require you to deliver your papers 72 hours in advance. It doesn't matter what CBSA says."