That was my first thought too. In order to go back to 2004 and every item you ordered on the internet would be daunting and a huge job. There would be no way that you could honestly complete this without either ensuring that you over estimate or you under estimate. Either way, you will not get it right.
UPS and FedEx know!
You were supposed to do this each year. It’s a lot easier to do it year by year.
Of course, it would be a lot easier on the residents of illinois if the companies collected the tax like the local companies do, but so far the feds have refused to grant that ability to states, so states are forced to burden their own residents with extra bookkeeping.