subscription for single journal issue

I have an online journal site; annual subscriptions are no problem, but I want to allow a user to pay to read a single issue; I have done this by creating a free registration plan that has a child plan as purchasable 'merchandise' allowing access to the issue chosen; however, when a user subscribes to this, pays via Paypal, the IPN is returned, payment is verified, the user is registered and can log-on, but the merchandise option remains unchecked and shows as an 'available upgrade'. Am I doing this wrong?

Merchandise plans can be re-purchased over and over. What you'll probably want to do is use a Subscription setting it to only allow single payments then set the number of occurrences to 1. This would allow them to only purchase it 1 time and its access protection would function like any other subscription plan. Also ensure you set the duration to lifetime and disable renewals.