Just out of curiousity, I went on the Garden Grocer site and picked out a few items I knew I would order if I were going to the Fort. I picked out just enough to meet the minimum order of $40. I then went on a local grocer site that has their prices online (not marked up or anything.) Surprisingly enough, my total on garden grocer was $42.01 and the local store was $39. The $39 also included two items that were on sale. I was actually pleasantly surprised by the Garden Grocer prices. The difference of course comes in the delivery fee of $12 and $1.99 fuel surcharge but I am thinking I would allow 2-3 hours minimum for a shopping trip so that is not so bad. I would rather hang out at the Fort and let them deliver. That $13.99 would be nothing compared to the mark-up at the trading posts even if you just needed drinks, snacks, breakfast foods, etc.
The sad part is, I realized how little you can buy for $40 anywhere. That wouldn't go far for a week at WDW for us. We could probably easily order the $200 to reach the free delivery threshold except that we really shop sales, etc. at home when stocking up for a camping trip.