I would appreciate any information one may have about financial trip-wires (taxes, tariffs, etc.)

Background: I've been developing a relationship with a company that is incorporated in the United Arab Emirates. The work will be technical writing for a short term contract, but my goal is to grow them into a regular client. They have an established history of working with companies in the UK and US, so I'm not too concerned about getting paid. I'm primarily concerned about the cost of doing business with the company.

I've researched tariffs and taxes, and so far have only found information about trading goods, not services.

I recently read a post here where someone was surprised by a tax situation after doing business in a state, and I'd like to avoid any surprises if possible.