Using Contact ID (from CRM) to get Cusotmer ID (from Invoice) isn't working

Using Contact ID (from CRM) to get Cusotmer ID (from Invoice) isn't working

Am I using the wrong url here?

https://invoice.zoho.com/api/v3/contacts/[CONTACTID]/?authtoken=[AUTHTOKEN]&organization_id=[ORGANIZATIONID]

I tried doing this with the Customer ID replacing contact ID and of course it works fine.... the problem being I have the contact ID and not the customer ID. 

I was told I could use the Contact ID to fetch the Customer object with the Invoice API.

What do?

**Edit: I need to run through a lot of contacts so I need to come up with a way to not use the SEARCH method. I'm trying to use the GET method.**

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