Although selling chocolates online are very important, advertising and customerís service is also important at Godiva.com
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.