So, the way to do this is through labels ('add label' option in the walker), basically when the bot reach a label it will do a certain action in the lua, so you need a label where you will be using the runes, and then waypoints all the way to the shop, a stand right by the shopkeeper, a label to buy the runes, waypoints all the way to where you want to use the runes, and repeat.
Now, for the lua, all you need to know is what functions to use, in this case its probably easier to just put the mana rune on the self healer and let it keep spamming it. I think you will only need these functions:
Self.ItemCount(ItemID)
Self.ShopBuyItemsUpTo(ItemID, Amount)
Walker.Stop()
Walker.Start()
In the tutorial section there's a simple tutorial that should be enough for you to understand.