That is definitely an option, especially for big expenditure that you're considering up front.
My thoughts might be too far ahead for this stage, but I would be inclined to batch my purchases up and then have a look at them all in one go, and just remember for the next time if there were any particular items to avoid. Obviously I'm not thinking about the exact steps about how I would enter all my purchases. I just assumed you'd sit down at your carbon calculator and put everything in once a week or so.
I'm thinking on the hoof but now it sounds like a good idea to have both ways of accessing the info - one for people like I describe doing their regular carbon audit, and one for people just visiting the website to check it out, or just considering something in particular.