As is, the kata seems to suggest that the tax is the actual tax due and simply asks you to add it to the subtotal. What you are actually giving is the tax as a percentage. The tax due needs to be calculated using the tax given as a percentage and the subtotal and only then added to the subtotal. Explaining this would make things much clearer.

I currently can't find another Kata on taxes using the search keyword "tax" but I'm pretty sure this topic of finding the total from the subtotal has already been done numerous times so this Kata is a duplicate. Please at least make a spin to this Kata to make it unique from the others.

Congrats on your first kata!
Just a few suggestions:
Round cost to two decimal places to represent the cents part(American money only?),
Insert commas above $999.99,
Require a dollar sign... like above...
No negative money... Are you paying the customer??
Hope to see more katas from you!!