WooCommerce API does not have a function to post advanced pricing rules. WooPOS only synchronizes the sale price (on the top of the special price rule editor) and start and end date.
You can use the third party plugins to handle the pricing online. But the prices won’t sync with WooPOS.
Another solution is to create a Sku for individual items, and another Sku for the package(case). You can setup two different prices for two Skus.