Creating an invoice through api end point

I would like to create an invoice from my application by requesting the Zoho Inventory create invoice api but I keep getting the following error:

{"code":4,"message":"Invalid value passed for JSONString"}'

I am posting with header content-type = 'application/json'
and the json body is as followed:
{
"customer_id": 1167451000000062085, //found by get request to get contacts
"invoice_number": "INV-00002",
"date": "2018-02-04",
"due_date": "2018-02-10",
"line_items": [
{
"item_id": 1167451000000062103, // found by get request to get items
"quantity": 130
}
]
}

Should this json be sent as query params and what is the JSONString with the data for post wriiten in the documentation?
Any help will be truly beneficial for me.

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