I noted that there is a bug in the pro sites paypal payment. In the pro-sites-files/gateways/gateway-paypal-express-pro.php file at line 1869 there is:
$nvpstr .= "&AMT=" . ($paymentAmount * 2); //enough to authorize first payment and subscription amt
When an user pay the subscription, the value considered is multiplied by 2.
Why is this value multiplied by 2?