I used Garden Grocer for our trip last May. I loved the service. I placed my order about 3 weeks out (they like to have as much notice as possible)
Yes, you can shop the site and save the order for placement later. I have already started my list for our upcoming trip in June.
The groceries were delivered on time, cold things cold, and with excellent service.
If you are not going to be there when they deliver than your hotel should be able to hold your items for you. I would double check with your hotel on this.
The prices are a little higher than maybe some grocery stores, but not outrageous.
I tipped the gentleman who delivered our order, even though they add a delivery fee.
It's a great service, and it beats having to pack things with you, or even make your own shopping trip. Who wants to stop at the grocery store when you're trying to get to the most magical place on earth!