This covers you when the client's campaign is a flop. However you do it, be sure to charge for it profitably. You should always have an agreement in writing. This should be updated each time you add to the service for that client. This agreement should clearly define the work that you are going to do, the reports that you are going to produce, the method and regularity of delivery of information, the cost of the service, and the payment terms.
Payment terms are very important.
You must not allow yourself to get into a position so that when the project is over the client has gathered all their money and you are sitting with a large receivable. If you have a particularly large account, it is not unreasonable to bill them every 15 days and give them 15 days to pay. Never extend credit longer than seven days beyond your terms. I have had more than one client who has lost thousands of dollars due to sloppy collections. I mentioned earlier that it is likely you will need a different software package to handle these calls because your TAS software is not designed to me the ETS market.