Adding K alone can result in a Ca and Mg deficiency, depending on the soil. Richard Campbell recommends 0-0-50. However, he's growing on very (very) calcareous soil. You're much better off applying a full nutrient package, unless you happen to live in Homestead. One year, I gave a healthy dose of K to my mangoes (without Ca), and jelly seed was off the charts.
The higher price for online ag products I was referring to excludes shipping cost. Margin is simply a lot higher for online ag suppliers, as they don't really cater to large scale growers.
Helena is one of the best deals around if you're close enough to get it delivered.