Hi,
That's indeed a major problem.
Once an order is made in HikaShop, HikaShop will trigger events. Joomla plugins can register these events to make actions based on these events.
You can read more about these here:
www.hikashop.com/support/documentation/6...mentation.html#order
So you could develop a plugin for HikaShop and in that plugin, you would implement the onAfterOrderUpdate event, check that the status of the order is being confirmed, and then you could contact the minecraft API to give the rank.
However, I don't know how to do that last part. You need to have a developer specialized in minecraft coding I suppose to help you with this.