I could not successfully sign up and the information on the site is very limited. What exactly does your service do? It implies all tax compliance, but I find that hard to believe. Perhaps you can confirm you comply with all the payroll requirements of the jurisdictions you operate in?

A US company that employs an individual resident in Canada is required to: - Keep W-8 on file - Register for a Canadian tax ID - Keep a "shadow" payroll system in Canada, and report monthly the payroll to the Canada Revenue Agency - Deduct Canada Pension Plan, Employment Insurance, and income taxes, and remit to the Crown (1) - Issue a T4 slip at the end of the Year (W-2 equivalent)

The above process is very different for a US company that employs an individual resident in the UK or France.

There are other factors to consider: residency status, hypothetical tax and equalization payments, and treaty relief comes to mind. I ask because, what we would call "Global Mobility Services" consisting of cross-border taxation of employees, is pretty much done in Excel because of the thousands of permutations possible with the above factors and 190 odd countries of the world.

(1) See Regulation 102 and Regulation 105 withholdings