Hi,
Well, if the plugin pay.nl developed doesn't send the email address of the user to pay.nl, it means that there is a problem in the code of the plugin. And so normally, it should be up to pay.nl to fix this.
Now, the code of the plugin for pay.nl can be look at directly on github:
github.com/paynl/hikashop-plugin/blob/master/paynl.php
There, you can see that the email address of the user is provided with this line of code:
'emailAddress' => $order->customer->email,
Checking your website, I can see that you've activated the guest mode in your checkout.
However, $order->customer->email refers to the email address of the Joomla user account of the current user. If the current user doesn't have a user account, that variable will be null.
That code should be changed to:
'emailAddress' => $order->customer->user_email,
The variable $order->customer->user_email refers to the email address of the HikaShop user entry. This will be set regardless of whether the user is a guest or has a Joomla user account on the website.