One of my Spanish software supplier offers half prices if the customer is outside the EU,
so I'm planning to open an LLC in the USA, with a bank account and po box address, but I'm wondering if there is a simpler way or intermediary services that can be used to achieve the same objective.
If your goal is to get a cheaper license, but use it within EU. Then it is possible what your trying to do is illegal depending on your country laws.
On an other side, it does not sound cost effective to setup a company in US just to purchase a software license for half price. In addition you would not be able to write it off as a business expense for your real company.