I would also think that if you were to monitor the number of pages worth of posts a user has this would be easy to detect from a non-moderator point of view. For example a script could check to see that UID 41911 (aka BadBear) has 162 pages of posts as of 12 Jan 2015, then if their post count went up by 100 posts in the following week then they should have exactly 167 pages of posts. If they do have as many pages then payment could be made automatically, if they do not then it would be an "exception" where posts could be counted manually
Wouldn't there be a big margin of error if you used pages to check for deleted posts though? That system might work for many deleted posts, but one, two, or even ten deleted posts might not show up if you checked the number of pages.
For example, if their post count went up by 115, you would expect to see 6 more pages. But if they had deleted 10 posts, it would still be 6 more pages. It's only once they delete 15 or more posts that the number of pages changes (to 5).