I am with you -- $10-15 tip seems appropriate. It's not like they are preparing your food and delivering to you table! They don't really even have any urgency with regards to delivering the groceries, since they already know in advance when you are arriving. Do the delivery people get paid waitstaff wages? I don't think so.
We have used Garden Grocer (GG) on 3-4 occasions. We love their service, but twice an item was missing and once an item came that we did not order. They remedied the omitted items the next day, but it's not like the tip helped with accuracy of the order. We will definitely use them again, because we like the service.
We have never used WGS, so I was just looking at their pricing structure. It seems a little high, compared to GG, although WGS will shop coupons and charges you the actual grocery receipt, and their delivery fee is their way of making money. I think GG up-charges items on their grocery list and has a flat fee for delivery. When we use GG, we generally hit the $200+ mark, so our delivery fee is only $2. With that, I don't mind giving a decent tip ($10-20). One other thing I like about GG is that there is a discount for ordering in advance. I think 10% at 60 days, 7% at 30 days. We generally order the same things every time, so we have a favorites list which makes it easy to place the order at 60+ days.
If you plan to use WGS, I would just put in a percentage that meets your preferred tip amount. If the order is $300, give 5% for a $15 tip. Although, when I was looking at their website this AM, it appears that a 10% tip is automatic with all orders left at bell services. We always have our order left at bell services by GG, even when it includes beer.