ActiveMerchant and PayPal Express Checkout

def checkout
  # ...
  if @order.valid?
    # Here's step 1
    response = paypal_gateway.setup_purchase(
      @order.total_in_cents,
      order_paypal_params(@order).merge(
        return_url: order_receipt_url, # Remember to use _url here and not _path (must be absolute)
        cancel_return_url: order_cancel_url,
      )
    )

    if response.success?
      @order.update(
        paypal_correlation_id: response.params["CorrelationID"],
        paypal_token: response.token
      )

      # Here's step 2
      return redirect_to paypal_gateway.redirect_url_for(response.token)
    end
  end
  # ...
end

def receipt
  # Here we're returning from step 3
  @order = Order.find_by(paypal_token: params[:token])

  catch(:failure) do
    if @order.payment_processed
      @error_message = "This order has already been paid for, no further action is necessary"
      throw :failure
    end

    # Here's step 4
    purchase_details = paypal_gateway.details_for(params[:token])
    unless purchase_details.success?
      @error_message = purchase_details.message
      throw :failure
    end

    # Finally, step 5
    purchase_response = paypal_gateway.purchase(
      @order.total_in_cents,
      order_paypal_params(@order).merge(
        token: params[:token],
        payer_id: purchase_details.payer_id,
      )
    )

    unless purchase_response.success?
      @error_message = purchase_details.message
      throw :failure
    end

    @order.update(
      payment_processed: Time.zone.now,
      amount_paid: @pack_order.total,
    )
  end
  # ...
end

private

def order_paypal_params(order)
  {
    items: [{
      name: "An Awesome Shirt",
      quantity: 1,
      amount: order.total_in_cents,
      description: "This is the coolest shirt ever, woo!",
    }],
    ip: request.remote_ip,
    allow_guest_checkout: true,
    currency: "CAD",
    invoice_id: order.id, # To make correlating easier later, if debugging
  }
end

def paypal_gateway
  @paypal_gateway ||= ActiveMerchant::Billing::PaypalExpressGateway.new(
    login: Configuration.paypal_username,
    password: Configuration.paypal_password,
    signature: Configuration.paypal_signature,
  )
end