I have not ordered a genny from Walmart online but have ordered maybe 10 / 15 other items, most for store pickup. I have been very pleased with the results. Their website is slow and needs work, and like someone said some items come from third parties. They are now offering a discount if you pick up in the store. It seems my GF and I are there every other day so that is not a problem.
I ordered two pair of jeans once and they only had one pair in Ark. center. They sent the other pair from Las Vegas and they both arrived the same day. Not very economical for them but good for me.