From my experience using eIS is the US seller might use USPS to ship it to their shipping hub but after that it usually goes through DHL and then the finally leg is done by Uni or Intelcom ( I live in Winnipeg).
I've purchased quite a few items from sellers using eIS and Canada Post hasn't delivered anything in a about a year.
It could also depend on where you live but eIS is well aware that Canada Post will be on strike.