Code for the Minimum Viable API Client (https://gelato.io/blog/minimum-viable-ruby-api-client-with-invoiced-and-httparty)

invoice_generator_client.rb

class InvoiceGeneratorClient

  PDF_CONTENT_TYPE  = "application/pdf"
  JSON_CONTENT_TYPE = "application/json"

  include HTTParty
  base_uri "https://invoice-generator.com"

  class << self

    def create_invoice(filename, options = {})
      response = post("/", body: options.to_json, headers: { "Content-Type" => JSON_CONTENT_TYPE })
      if response.success? && response.content_type == PDF_CONTENT_TYPE
        save_pdf(filename, response.parsed_response)
      else
        # something has gone horribly wrong!
      end
    end

    private

      def save_pdf(filename, data)
        File.open(filename, "wb+") do |f|
          f << data
        end
      end

  end

end

payment.rb

  def generate_invoice
    InvoiceGeneratorClient.create_invoice(
      Rails.root.join("tmp", "invoice-#{id}.pdf"),
      from:   "My Company, Inc ®",
      to:     customer_name,
      logo:   "http://placekitten.com/320/140",
      number: id,
      date:   created_at.to_date.to_s,
      items:  [
        {
          name:      description,
          quantity:  1,
          unit_cost: self.amount_in_cents / 100.0
        }
      ],
      notes: "Thank you so much for your business!"
    )
  end