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Can I setup Zoho Billing - Zoho CRM integration with Free/Standard edition of Zoho CRM?
If you have Free/Standard edition of Zoho CRM, you can only enable Contacts sync through the integration.
But, if you have a Professional/Enterprise edition, you will be able to sync Products, and have Custom modules enabled in Zoho CRM. You can also enable workflow triggers when you are in Professional/Enterprise editions in Zoho CRM.