Although selling chocolates online is very important, advertising and customer service is also important at Godiva.com[/quote]
Note, the subject is 'selling chocolates' (it), so it requires a singular verb (is):
It is very important.
What is very important?
"Selling" functions as a verbal noun, a gerund, and it takes as its object the noun "chocolates". "chocolates" is not the subject, so the verb shouldn't be plural "are".
Also note, "customer service" is a noun phrase. "customer" functions as an adjective modifying the noun "service". It tells us what kind of service.