A superset of contractor, employee and businesses?

I need a single word to define an employee, contractor or any business entity (e.g. sole-proprietorship, corporation) that provides services in exchange for pay.

I saw https://english.stackexchange.com/a/14187/152901 but the terminology is too vague for my liking.

https://english.stackexchange.com/a/248668/152901 suggests "Vendor" which comes close to what I'm looking for, but I don't believe the definition covers employees.

UPDATE: I am looking for a word which is the union of a "Vendor" (which refers to a company) and "Worker" (which refers to an individual). Ideally the word should be stated from the perspective of the individual/company (what would they call themselves?) and should imply some sort of business relationship.

Since employees and independent contractors are handled differently in taxation in the U. S., one receiving a W2 and the other self-employeed, one usually having taxes withheld by their employer and the other having to pay self-employment tax (Social Security) in addition to income tax, etc., there is no single word I know of that would accurately combine the two, let alone include other vendors like small businesses and corporations as well. @Jim suggested the reasonable superset term, "Taxpayers," which implies the one thing the components have in common with regards to a tax simulation program. My wife, a retired accountant with more than 35 years of experience in corporate, partnership and individual tax preparation, could not think of a better word that would include all the entities that will need to use your program.

Edit 1-3-16: @Gili, After thinking more about this for a few days, I came up with one other possibility that might work for you: "Client."