direct purchase without a prior discussion.

I have been providing Virtual Assistance services here on different niches to businesses. Being a broad niche have worked on variation of tasks. My question is, what if someone directly purchase my gig without any discussion about the task and it's volume. As we only have to wait for the requirements submission from buyers and the order starts. Many cases, I got an overload of tasks that don't match with my gig price. What can I do in those case while order cancellation is never a good decision. 

Thank you in advance.