Creating an Estimate using PHP/cURL problem

Hi, I've trying to debug a problem adding an estimate. My PHP is below, I've reformatted it a bit to make it simpler to read, the code is only a proof of concept. This generates a HTTP 400 response indicating a bad request or bad ID.

It's based on this: https://www.zoho.com/invoice/api/v3/#Estimates_Create_an_Estimate

Any help would be appreciated!

Thanks,

Niall

$access_token = MYACCESSTOKEN;

$organisation_id = MYORGID;

$estimate_request = [

"customer_id" => 4780638000000343007,

"estimate_number" => "EST-00002",

"reference_number" => "TEST-12346",

"date" => "2021-04-13"

];

$data = 'JSONString="' . json_encode( $estimate_request ) . '"';

$headers = array('Authorization: Zoho-oauthtoken ' . $access_token,

'X-com-zoho-invoice-organizationid: ' . $organisation_id,

'Content-type: multipart/form-data');

$ch = curl_init();

curl_setopt($ch, CURLOPT_URL, "https://invoice.zoho.com/api/v3/estimates");

curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);

curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);

curl_setopt($ch, CURLOPT_POST, true);

curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, FALSE);

curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, FALSE);

curl_setopt($ch, CURLOPT_FAILONERROR, true);

curl_setopt($ch, CURLOPT_POSTFIELDS, $data);

$result = curl_exec($ch);