I received an e-mail today from a social media marketing agency telling me about a competition that's going to be held for one of their clients, in the hopes I will post a story about it on my website. This is my first offer, so i'm not entirely sure how I should proceed. As in should I charge a fee? And if yes, the fee would differ for each site depending on popularity. How do I establish how much I should charge? Any advice would be greatly appreciated.