Perroow hey there! Gotta love those tax challenges said no one ever! Like you mentioned, you'll need to speak to a tax authority before proceeding, but I can tell you how it worked from my experience...
Canadian webshop, a Chinese supplier, USA end customer -- very similar, just different country that the business originates from. In this case, you can take Spain out of the equation. Generally, in the US, unless you have what is known as a tax nexus (warehouse, employee, physical presence) in the state that you are selling to, then you do not charge taxes.
Fortunately out-of-the-box, Shopify is set up to collect taxes automatically based on three levels of taxation in the US. Given that you do not have a physical presence in the US, likely no taxes will be collected. Beyond that, you simply report income in your country in compliance with your local tax laws.
Hope this helps! And as always, feel free to ASK the community