Help with Paid Classified Ads site

I´m trying to develope a Paid Classified Ads system on Drupal 6.

The scenario is:

1. There are different types of ads, and you have to pay to publish one.
2. There are different prices depending on the time you want to keep the advert published.
3. At any time you can extend the time, "buying more time".

I´ve searched in the Forum, I saw a couple of modules for Ad Systems but no one does everything I want. Point 1 and 2 are resolved using UC Node Checkout and CCK and some programming to get the node published when the user pays. I´ll also have to use Rules to check if the advert has expired in time.

Node Checkout resolves the issue with paying for publishing a node. The problem I have now is point 3: how can I use the Ubercart module to associate a product with the extension of the time.

I imagine the solutions consists in using Views combined with a product; Views to get a list of the published nodes and select which one you want to extent the period, and write some lines to change some data in the specified node after checking out. But, I don´t know if this is possible.

Can anyone help? Any ideas.

Thanks!