PayPAl Express without SSL

Hi,
I'm using the plugin from http://noppaypalexpress.codeplex.com/ and NOP Commerce 3.0. I used to be able to use it without SSL, but now I can't.
It happened after I configure PayPal Direct and PayPal standard to use instead. Now I want to go back to Express Checkout, but every time I'm trying to go to checkout, I get this error message:

We apologize, but an error has occured.
11813 | Callback URL is wrong type. | Callback URL is wrong type; you must use the HTTPS.

Adding SSL unfortunately is not an option. What should I modify in the source code to make it working? I know that in Magento there is this "Transfer Shipping Options" option that makes it possible to use Express Checkout without SSL, but I can't find anything similar in this plugin.

I have same error when the language English is selected, but when i change's language for French or Spanish, i have this error:

Error: System.FormatException: Input string was not in a correct format. at System.Number.StringToNumber(String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal) at System.Number.ParseDecimal(String value, NumberStyles options, NumberFormatInfo numfmt) at System.Decimal.Parse(String s) at Nop.Plugin.Payments.PayPalExpress.PayPalExpressCheckoutPaymentProcessor.SetExpressCheckout(PayPalProcessPaymentRequest processPaymentRequest, IList`1 cart) at Nop.Plugin.Payments.PayPalExpress.Controllers.PaymentPayPalExpressController.SubmitButton()

NOTE: I working with 2.8 version from nopCommerce.