My last thirty years working I operated a handyman service repairing condos. Replacing fence posts was one of the most common jobs and I'd had plenty of experience to refine the procedure. I welded up a paddle, much like a drywall mixer, and used a plastic five gallon pail. Put in one half of the bag, with all the water, mix, and then add the rest of the bag, mixing a bit more. It was easy to pour out the mix and easy to clean afterwards. Saving a few minutes a post, and reducing the effort made a big difference when repairing a mile of fence. If the situation allowed, using a trailer with gravel and portland cement saved a lot of money, making a noticeable difference in the profit.