In spamgourmet, you can set an “authorized sender” address for each alias (only one, alas); messages from the “authorized sender” are not counted against the message count, but all others are. This should resolve your problem in at least some situations.This sounds interesting! It seems like it would be a lot of work, though, for a site that you might want to hear from regularly. I would worry that I may miss an email whose alias I forgot to "extend", or from a sender who suddenly had a valid reason to increase frequency.
While it's true that spamgourmet may not be a good fit for such cases, I'm not sure how many senders I have that absolutely would be good candidates for it, apart from maybe the extremely rare truly one-off situation.