what should i be paid to promote a business online?

[P.]

Your duties and responsibilities should be outlined in your contract. You built their web site and maintain it. Usually that ends your responsibility and you should be charging a monthly fee for this service. If you are cross posting to social media sites, you are also doing their marketing and should be included in the services you offer in the contract. Once you start answering their emails and partaking in the running of their business, you are now essentially an employee, not a vendor. The company is saving money on an employee using you to answer emails and customer inquiries.