You could work around this by generating some sort of really long hash of a secret cookie and the sending domain name, then providing that hash to the sending website. When sending mail, the sender could send it to you at youraddress+hash@yourisp.com -- Your ISP would then know that you'd granted that site permission to mail you for free and wouldn't charge them.
Ideally, you'd be able to go to a page on your ISP's site and revoke these revocations for free mail at some point if the site started abusing the privilege.
Alternately, you could just enter a list of domain names from which you'll accept mail without charge - combine this with SPF and you'd be a fair bit of the way there.