How to track a purchase made outside of infusionsoft but from a campaign?

I am needing to pull contacts out of a campaign if they purchase a product that is not inside of infusion soft. I use teachable as my purchase platform and am running a free trial to a product. If they purchase the product I need to remove them out of the campaign, but not sure how to track that the purchase actually came from the specific free trial campaign.

I am pretty familiar with zapier and my first thoughts were to use zapier to say if they purchased and had “this specific tag” (which is a tag that lets me know that they did access the free trial), then apply “this tag”(a tag that says they enrolled from the free trial) and this would move them out of that campaign. Unfortunately, zapier did not have a filter that allows finding a contact by a tag in infusionsoft. It only finds a contact by email address or username.

I could use a product purchase goal, but my products are sold outside of infusionsoft on another platform, so I’m not sure how to do this. Any ideas would be helpful.