And the additional delivery charge makes it even more. However, I don't think you tip based on a percentage of the total amount for a grocery delivery. To me it would seem that it would be based on the quantity and maybe how heavy some items might be. Someone who orders several cases of water, bottles of bleach, laundry soap, etc. might not spend as much as someone who was getting a bunch of produce, but the produce would be lighter and easier to handle than the water and other things.
I am sure the 3rd party people make more than people who work at Walmart. Walmart is notorious for not treating their employees well. My stepson used to work there and they would never let him get more than 32 hours so that he would not be able to get benefits. Did you know that they offer classes to their employees on how to apply for food stamps and government assistance? And they got hit hard with a lawsuit a number of years back for gender discrimination against women.