How bout just using 22.4's the way they were originally designed. Work inside, and only drive when needed to reduce requested RPCD overtime and Saturday. A Tue-Sat schedule to deliver the Saturday volume.
Pay them full driver rate when they drive, and if they drive more hours than a set limit, more than just to reduce unwanted OT and Saturday delivery, force the Company to hire another RPCD.
If they drive a few hours a day and on Saturday, fine. If they're used to drive all the time, have a contractual clause to hire another RPCD to do that work that the 22.4 has been doing.
And this way, at least the Company is not using a "cheaper" driver to deliver their packages. The 22.4 will be full driver rate.