Customer after delivering a product: this is not what I asked for? how to avoid such problem?

famous problem, you can have you customer as a team member, so he/she will always knows what will be delivered this is from extream programming. also, you can always deliver every two weeks, so customer will always have a product, and will not wait to the end to see a product.

1- Capture the customer request accurately and make sure to get his approval in writing on the product specifications before starting;

2- Share prototypes, milestones and ensure the end product is clear for both of you.

3-Compare end product to prior specifications and provide customer with final check list.

You should have System requirements and specifications document ready and got it agreed at the time of finalising the project deal.

To avoid, have the SRS, and functionality requirements documents ready and get it agree and finalise between them.

If agreed, but not provided the functionality means it's the liability from the solution provider side. otherwise its a new functionality can be explained to customer.