I am a web developer/designer and I constructing my plan/strategy on the process on delivering a project. My problem is what happens after the delivery.
As far as I know I have to send a plan/aggreement to my customer, to inform him how the page will look like, how it will work, time needed and the cost. After a discussion, assume that they deal is on.
Suppose that everything goes fine, the project is delivered and running. It is very common thing that after a period the customer may come back and ask for a change (small or big), an addition, something extra.
My question is what should I include in the agreement about the after-delivered period? Should I tell that any changes after delivery will have a cost ? Should I avoid it and have a new agreement about the extra thing? How to move in this stage?
Assuming you have a written agreement with the client, any requests after delivery that were not part of the original agreement should be treated as a separate agreement. Create a new contract/agreement for the new requests and charge accordingly.
If the request is for something you agreed to deliver, but failed to, then obviously you should complete it at no charge.
If possible, making it a time & materials contract can work very well for both sides (ie. charge per hour, not a lump sum for the project). This means they can have as many changes as they want, but they pay for them. A lot of clients won't accept this as they don't trust you, which is unfortunate, as it can be in there best interest too.
As for how to deal with late changes on a set project, you should have an acceptance form that they sign to say the work is complete. Any changes after that are outside the scope of this project and to be dealt with separately. You could provide your normal hourly rate to be applied for minor changes later.