Christ, if you want to prevent any sort of problems after the contract begins, I would prefer it if you would evaluate everything before you accept the offer. This is because it will help you avoid any type of disagreements after the contract starts. The nature of the task, as well as the amount of money the client will pay you, etc.
Always keep everything on Upwork Chat, and write before accepting the offer. Describe the strategy you intend to use for that particular offer. In the case that the client requests more work, you should get in touch with the Upwork support team as soon as possible; I'm certain that Upwork will be able to assist you.