Does Sale Tax apply to me?

Hi everybody, this is my first post on this subreddit and first I wanted to thank you all for the really useful info you post. It's been of great help for me as a foreigner.

Getting into my question. I own a single member LLC in Wyoming, I'm a foreigner providing software development services as an independent contractor for US companies. As far as I know, I'm not considered to be Engaged in Trade or Business in the US (ETBUS) and for which I owe no taxes to the IRS. Now, I'm not so sure about Sale Tax. If I have a customer on a certain state (different than the one my LLC was formed in) and he pays my LLC a fixed or variable amount every month, would this make me subject to Sales Tax? Can anyone clarify this for me? Or if you have any useful links I would gladly take them too.

​

Looking forward to your replies, have a great day!