code: def set_payg_billing(currency, can_purchase_credits, client_pays,
        markup_percentage, markup_on_delivery=0, markup_per_recipient=0,
        markup_on_design_spam_test=0)
        options = { :body => {
          :Currency => currency,
          :CanPurchaseCredits => can_purchase_credits,
          :ClientPays => client_pays,
          :MarkupPercentage => markup_percentage,
          :MarkupOnDelivery => markup_on_delivery,
          :MarkupPerRecipient => markup_per_recipient,
          :MarkupOnDesignSpamTest => markup_on_design_spam_test }.to_json }
        put 'setpaygbilling', options
      end
docs: Sets the PAYG billing settings for this client.