I am struggling with why/how the hourly compensation plays into your tipping. You don’t ask servers at a restaurant what their hourly compensation is before you decide to tip, do you?
If the order is correct, undamaged, and delivered at the specified time, add a tip (I think $10-15 seems reasonable for a good sized order). Your saved time and decreased stress of not having to shop at a Wal-Mart is worth at least that, right?