Just to followup...with help from ConvertKit support and Zapier support I solved the problem using Zapier. When there is a new charge in Stripe, a zap is triggered to get the name, email address and tag name from the charge (the tag name is the name of the product sold). After a little prep, the tag id is added to the subscriber. As @Paris Hansch48 suggested, an automation uses the "subscriber being added to the tag" event to trigger the "subscriber being added to a sequence" action.
The only downside is that I'll need to use the paid version of Zapier to use this zap because the zap contains more than 3 steps.