I've tried the following, updating all of the "can_send_in_mail" ...
invoiceID = invoice.get("invoice_id");
fullInv = zoho.books.getRecordsByID("invoices", organization.get("organization_id"), invoiceID);
inv = fullInv.get("invoice");
docs =inv.get("documents");
for each find in docs
{
find.put("can_send_in_mail",true);
}
newDocs = Map();
newDocs.put("documents", docs);
newDocs.put("can_send_in_mail",true);
response = zoho.books.updateRecord("invoices", organization.get("organization_id"), invoiceID, newDocs);
info response; // shows true but not enabling the document option as in image ?
See attached....