Marketpress Subscriptions and Inventory

Howdy. I'm using Marketpress to do subscriptions. I'm able to do this with the External Link, Paypal work around. I'm also trying to use the Marketpress inventory feature. However it doesn't seem to be working.

I suspect that Marketpress can't track inventory on items sold with the External Link. Am I correct in my assumption?

Thanks for taking the time to look at this.