maybe both @everyone and @here can be used for all users in the forum and all users in the topic, respectively. (is it possible to make it available for privileged users only?)
@oplik0 's plugin will come in handy for restricting those usernames 😆
@julian thank you for your answer. I agree that stripe is probably better siuted for this, the only thing is, that here in germany the usage of paypal is much bigger than credit cards, which is the primary payment method for stripe i think.