I want to use ubercart instead of signup.

This article makes some good arguments for why this is so hard now- ubercart specific information is not available to the rest of drupal through rules.

http://ocdevel.com/blog/event-signups-registration-ubercart

An excerpt:
Signup can send out reminder emails:
--"When used on event nodes (with event.module installed and regular cron runs), it can also send out reminder emails to all signups X days before the start of the event (per node setting) and auto-close event signups X hours before their start (general setting)."

NOT doable (with Ubercart). I initially thought that this functionality would be doable via Rules; however, as of Drupal 6, the workflow-like triggers & actions for all Ubercart data is handled directly by Ubercart, and is not exposed to Drupal. Without this data, neither trigger.module or Rules have any conditions, triggers, or actions to work with. So while Rules can handle time-based workflows, the necessary data isn't exposed by Ubercart, rendering this functionality impossible.