When you close your sales deal,what more do you sell to customers?

Before to start / think selling more things to your client, make sure that he is totally happy with the recent provided one, and you have been supportive and good advisor to him.
Once you are sure of reaching that level of service, then you need to work on increasing the good relationship with the client in a friendly way also in business side as well.
Then you can start offering the related services or extra benefit for him as an advice or addtional related feature or package.
So we sell: High-level-service or Valueable Product, Trust and Additional Services / Product.