Simple REST Client for the Zoho Invoice API, using HTTParty. You can search for customers, create a new one, create a new invoice, add a payment and send it.

zoho.rb

# Copyright Applidget - 2013
# Author : Sébastien Saunier (@ssaunier)
# Released under MIT License

require "httparty"

class Zoho
  include HTTParty
  base_uri 'invoice.zoho.com:443/api'
  
  PAYMENT_METHODS = {
    1 => "Check",
    2 => "Cash",
    3 => "Bank Transfer",
    4 => "Paypal",
    5 => "Credit Card",
    6 => "Google Checkout",
    8 => "Authorize.Net",
    9 => "Bank Remittance",
    10 => "Payflow Pro",
    11 => "Stripe",
    12 => "2Checkout"
  } 

  # Zoho constructor
  #
  # api_key: One time process. A customer must request the API key by registering at https://zapi.zoho.com/apigen.do
  # auth_token: Customer must go to https://accounts.zoho.com/apiauthtoken/create?SCOPE=ZohoInvoice/invoiceapi to generate it.
  #
  def initialize(api_key, auth_token)
    @auth = {
      :authtoken => auth_token,
      :apikey => api_key,
      :scope => :invoiceapi
    }
  end

  # http://www.zoho.com/invoice/api/customers/customers.html#how-do-i-find-a-customer?
  #
  # Will return an array of hashes containing a customer.
  def customers(search)
    raise ArgumentError.new("You must provided a non empty search token") if search.blank?
    result = self.class.get '/view/search/customers', :query => @auth.merge(:searchtext => search)
    result["Response"]["Customers"]["Customer"] || []
  end

  # http://www.zoho.com/invoice/api/customers/customers.html#how-do-i-add-a-customer?
  #
  # Options to provide:
  #   - Name (mandatory)
  #   - PaymentsDue (e.g. "0")
  #   - CurrencyCode (e.g. "EUR")
  #   - BillingAddress, BillingCity, BillingZip, BillingCountry, BillingFax
  #   - Contacts => Array of Contact:
  #     - Salutation, FirstName, LastName, EMail, Phone, Mobile
  #
  # Will return a couple:
  #   1. Hash containing the created customer, with id at key "CustomerID"
  #   2. Error message if first element is nil because it could not be created.
  def create_customer(options)
    create(:customer, nil, nil, options)
  end

  # http://www.zoho.com/invoice/api/invoices/invoices.html#how-do-i-add-an-Invoice?
  #
  # Options to provide:
  #   - InvoiceItems => Array of InvoiceItem
  #     - ItemName
  #     - ItemDescription
  #     - Price
  #     - Quantity
  #     - Discount
  #     - Tax1Name
  #     - Tax2Name
  #   - Comments => Array of Comment:
  #     - Description
  #   - Notes
  #   - Terms
  #
  # Will return a couple:
  #   1. Hash containing the created invoice, with id at key "InvoiceID"
  #   2. Error message if first element is nil because it could not be created.
  def create_invoice(customer_id, options)
    create(:invoice, :customer, customer_id, options)
  end

  # http://www.zoho.com/invoice/api/payments.html#how-do-i-add-a-payment-to-an-invoice?
  #
  # Adding a payment to a method will automatically change its status from "Draft" to "Open".
  #
  # Options to provide:
  #   - Mode (1->Cheque, 2->Cash, 3->Bank Transfer, 4->PayPal, 5->CreditCard, 6->GoogleCheckOut, 7->Applied from Credit, 8->Authorize.Net, 9->Bank Remittance)
  #   - Description
  #   - Date (format: "yyyy-mm-dd". If not given, today's taken)
  #   - Amount (float)
  #
  # Will return a couple:
  #   1. Hash containing the created payment, with id at key "PaymentID"
  #   2. Error message if first element is nil because it could not be created.
  def create_payment(invoice_id, options)
    create(:payment, :invoice, invoice_id, options)
  end

  # http://www.zoho.com/invoice/api/invoices/invoices.html#how-do-i-send-an-invoice?
  #
  # Options to provide
  #   - Custom.Subject
  #   - Custom.Body
  #   - ToEMailIDs  (emails, comma separated)
  #   - CCEMailIDs
  def send_invoice(invoice_id, options)
    options.merge! :InvoiceID => invoice_id
    self.class.post '/invoices/send', :body => @auth.merge(options)
  end
  
  def get_invoice(invoice_id)
    result = self.class.get "/invoices/#{invoice_id}", :query => @auth
    result["Response"]["Invoice"]
  end 

  private

  def create(type, parent_type, parent_id, options)
    object = parent_type.nil? ? {} : { "#{parent_type.capitalize}ID" => parent_id }
    options.each do |key, value|
      object[key] = value unless value.blank?
    end

    body = @auth.merge :XMLString => object.to_xml(:root => type.capitalize)
    result = self.class.post "/#{type}s/create", :body => body

    # Zoho API is really poor at handling HTTP status codes. 200 is returned
    # when the object was not created because of an error.

    return result["Response"][type.capitalize.to_s], result["Response"]["Message"]
  end

end

This is latest api documentation:
https://www.zoho.com/invoice/api/v3/#introduction
and no need to create apikey from zapi.zoho.com