Do you "need" to go that fast or are you just doing it because you know you'll be tired and want to shorten the trip? With Auto Pilot, that problem goes away so why not leave 15 minutes earlier, go 70MPH in Auto Pilot and enjoy the ride?
The air conditioning doesn't use as much as you might think. Maybe 2 to 3 miles per hour in recirc mode. So, with 4 hours of driving, you're looking at a loss of maybe 6 miles. You can easily regain that by slowing down.
Regarding the charge to 100%, if you only do it once a week, and you drive it shortly after charging is complete, it's not a problem.
If you decide on the 75kWh, you'll know when you arrive at your store the first time how much range you used. If less than half, you'll know you can get home (account for a buffer of course). If not, then stop at the Supercharger for 5 or 20 minutes. Is that worth $10K?Probably.
With all of that said, you will definitely be happier with the 90, but I think the 75 will work for you.