When I try to test the Zap I'm receiving this error: JSON is not well formed.

I've contacted the Zapier support team and they told me this:

Jumping in to help here. I believe the issue lies with this field here: https://zapier.cachefly.net/storage/photos/37b3d554e31ccaa29de14d21ba6727a6.png

You'll need to sub "TAX ID" for the index of the custom field in Zoho Subscriptions. This is likely an integer that refers to the TAX ID custom field. I couldn't find any documentation on finding the index oh Zoho Subscriptions site so you may need to ask their support for help there.

Let us know what you find! 

What to do to save the TAX ID on the customer profile through Zapier?