Btukfyl

Why is the noun company here followed by the verb in plural? Does this actually mean - one of the companies, and is it how it should always be written ?

"... to use the services of a web hosting company who charge a fee..."

American English usually sees company as a singular collective noun, and thus uses a singular verb. This flavor of English also tends not to use the personal who, but the impersonal that or which — in fact, Grammar Girl insists on it.

British English regularly sees company as an aggregate of individuals, thus with a plural verb:

We can book a drug test for you with a local company who charge $60 … indeed.com – New Zealand

If you do go for full insurance, don't buy it through the car hire company who charge £12 a day or more … — Guardian, 24 May 2018.

All services related to your power connection and your meters are handled by your distribution company who charge us regardless of whether you're a new or existing customer. — Powershop.com.au, 7 Sept. 2016.

Guernsey Water are the only company that charge customers with cesspits/septic tanks … Manx Utilities

Note that company as plural also goes for the proper names of businesses. American English almost never does this.

This does not mean that an American does not occasionally use a plural verb with company:

With high ceilings and hundreds of thousands of books neatly tucked away into boxes, the facility itself is privately owned and operated by the Clancy-Cullen Moving and Storage _Company, who charge as much as $13 to transport each volume. — Miscellany News (Vassar College), 144/18, 24 March 2011.

Since the company name consistes of two surnames, the writer was likely led to use a plural verb as well as the personal who: Clancy and Cullen are the ones charging the $13. Otherwise, North Americans generally go for the singular, even with who:

We have found another company who charges less for their services. — Technology Initiatives Group Ltd. (St. Catherines ON)

There is no question that you will get out of debt cheaper with a debt settlement company who charges a percentage of savings vs. a percentage of total debt. — Trident Debt Solutions, Boulder/Denver CO.

So whether you use a plural verb depends mostly on which standard of English you wish to follow.