The "integration" they have currently is a seperate "zoho finance" tab within the CRM that can't talk to any of the native CRM moduies to pull their information (sales order details, invoice details, etc) and appears to be just by a skin/input area to create a new zoho books invoice from within the CRM application. This was confirmed by the ZohoBooks chat support team. 

SO, i'm wondering, is there a way to use a button that invokes a custom function to pull the sales order information (saels order number, billing details, product details, etc.) from the native CRM sales order module and use that information to pre-populate / create an invoice in the ZohoBooks application? Seams like you could either invoke a URL with url parameters, or run a function that stores that information in the browser window temproarily then populates the fields onze ZohoBooks invoice screen opens.