We are trying to integrate zoho books with our application (not integrated with zoho in any way), and create invoice for orders generated in our application.

To create an invoice, the customer_id is a mandatory field, so we will have to add our customer as a 'contact' before we can generate an invoice for that customer.

We can add a customer as a contact using APIs as well. But how to check if a customer has already been added to zoho books ?

All the API calls to check if a customer exist depends on the 'customer_id' of that contact. Is there anyway to check if a contact already exists using any different field like name, email, etc.. ?