What are the customs and laws which define the relationship between the client and service provider?

A services contract whether international or domestic is the foundation of this relationship . 

It usually depends from a country to another.In general relationship between the client and the service provider is based on some commonly standards:

1. Acceptance & Acknowledgement of both parties.

2. The reserved rights for both parties

3. Delivery of right service or product

4. Obligation of payment once service is received

... etc

Mutually rewarding benefits: A right balance between profitability and customer/client satisfaction.