$ curl https://books.zoho.com/api/v3/invoices?organization_id=10234695 -X POST -H "Authorization: Zoho-authtoken ba4604e8e433g9c892e360d53463oec5" -H "Content-Type: application/x-www-form-urlencoded;charset=UTF-8" -F 'JSONString="{ "customer_id": 982000000567001, "contact_persons": [ "982000000870911", "982000000870915" ], "invoice_number": "INV-00003", "place_of_supply": "TN", "gst_treatment": "business_gst", "gst_no": "22AAAAA0000A1Z5", "reference_number": " ", "template_id": 982000000000143, "date": "2013-11-17", "payment_terms": 15, "payment_terms_label": "Net 15", "due_date": "2013-12-03", "discount": 0, "is_discount_before_tax": true, "discount_type": "item_level", "is_inclusive_tax": false, "exchange_rate": 1, "recurring_invoice_id": " ", "invoiced_estimate_id": " ", "salesperson_name": " ", "custom_fields": [ { "customfield_id": "46000000012845", "value": "Normal" } ], "project_id": " ", "line_items": [ { "item_id": 982000000030049, "project_id": " ", "time_entry_ids": [ {} ], "product_type": "goods", "hsn_or_sac": 80540, "warehouse_id": "", "expense_id": " ", "name": "Hard Drive", "description": "500GB, USB 2.0 interface 1400 rpm, protective hard case.", "item_order": 1, "bcy_rate": 120, "rate": 120, "quantity": 1, "unit": " ", "discount_amount": 0, "discount": 0, "tax_id": 982000000557028, "tax_name": "VAT", "tax_type": "tax", "tax_percentage": 12.5 } ], "payment_options": {}, "allow_partial_payments": true, "custom_body": " ", "custom_subject": " ", "notes": "Looking forward for your business.", "terms": "Terms & Conditions apply", "shipping_charge": 0, "adjustment": 0, "adjustment_description": " ", "reason": " ", "tax_authority_id": 11149000000061052, "tax_exemption_id": 11149000000061054, "tax_id": 982000000557028, "expense_id": " ", "salesorder_item_id": " ", "time_entry_ids": [ {} ], "payment_gateways": [ {} ], "gateway_name": "paypal", "additional_field1": "standard" }"'

The response is:

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

Of course I tried with my own OAuth value too.