Hi,
I don't see why that would happen.
The coupon value is supposed to be sent to PayPal too.
I wanted to check on your website, but your products have a user group after purchase set, which requires to be logged in with a user account, but your checkout is configured to only allow guest checkout so I'm not able to move forward the login step of your checkout.
An easy solution however is to turn off the "send order details" setting of your PayPal payment method. That way, you'll get the full amount of the order sent to PayPal instead of the details and the amount will correspond to the order total amount, no matter what.