Hi Gilden,
very nice from you! I have couple of ideas:
1) User must not be able to re-publish (extend) the ad before the expiration date&time has been reached. This is in order to avoid constant daily renewals (system abuse) and uneccessery 'spamming'.
2) If not too complicated, renewed ad should be counted as a new published ad (value added to the total number of already published ads and than that number substracted from total amount of allowed ads per, say, month) (edit: this step is redundant). I hope this will not affect this plugin, say if in a future Osclass gets an update with options for basic and premium users (different amount of ads per month to avoid spamming/flooding the ads sections).
Thanks!
dev101