I like the Target/Shipt method. After the items are delivered, you can an email with an option to tip. Allowable tip options are 10, 15, and 20 percent, or a custom amount. I first make sure that all the items are correct and I weigh the service/communication of the driver before I tip.
I take also into account what I ordered. If I ordered just a few boxes of diapers, I often give a flat $5 tip. Why should the driver bank on my 10 or 15 percent on expensive items when there was so little effort. On the other hand, when I stock up on drinks, I often tip 20 percent, because the driver is carrying very heavy but inexpensive items.