Each contact should be able to view all the estimates / invoices they are associated with, because once they start to be a part of different projects with different clients, we will not be able to have so many email ids for one contact for them to view all estimates / invoices etc. they are associated with.

So when a contact is associated with many different estimates / invoices etc. he / she will need only one email and login to access the portal and view all associated estimates / invoices etc.

Alternative on the same lines I suggest the following:

1. Create a Project, for example - Project A
2. We will add, Client Details, Billing Address, Shipping Address, email Id, tel number etc. (this could be an existing client in my contacts)
3. We will also add other contacts associated with this project (could be new or they could be existing contacts in My contacts)
4. We can then create estimates / invoices related to this project and its details.