I used to live at Grandville and Kenmore, years ago. I would suggest that you can take the Red Line down to Foster, the Foster bus to Jefferson Park (First Train is at 8:48AM), and there catch the Metra (not the CTA Blue Line) out to Mount Prospect (
Metra Union Pacific Northwest Line). Alternately, you can take the Red Line downtown, then walk over to the Ogilvie Transportation Center to catch the Metra out to Mount Prospect (First Train is at 8:30AM). the Mount Prospect Metra Station is about a hundred and fifty paces from the store. You can get a Metra Train schedule
here (the store is south of the tracks and to the west of the station, the green star, at the intersection of Wille and Prospect Ave). It's the same train, of course, and the bus ride might be less distance but more time (and more likely to run into difficulties as the Foster bus basically starts out at the Red Line, Thorndale stop, I believe, and ends at the Jefferson Park Station - the longest possible bus ride). I'd stick with the Red Line downtown and save the hassles of transferring. It'll take about twenty minutes to get from a downtown stop to Ogilvie Station, on Madison, if you walk it.
You can work out your schedule to get to either the Jefferson Park Metra Station or the Ogilvie Transportation Center by using the "Trip Planner" on the COT website
here. Just load your address and the name of either station and it should give you some alternatives. Be sure to check the Metra Schedule first for the departure time from both stations so that you can include it in the trip planner. Good luck!