UPS trucks carry lots of packages so that they don't have to reload at the warehouse mid-day. That's not really a concern for Walmart employees, since they wouldn't be making any special trips to the store to load up on packages.

True, UPS has the advantage of denser delivery routes. But I'm sure Walmart has done the analysis and determined that they can deliver (some) packages more cost-efficiently than UPS despite this. Same with Amazon, which has been hiring their own couriers in some areas.