Memberships at a discounted rate requires that automatic Re-bill be turned on for your account at the regular rate.
You can cancel the automatic renewal by going to Account > Billing > Stop Automatic Re-bill after 48 hours of the purchase. If you do not see the button to cancel the auto Re-bill it's because your most recent charge has not finished processing yet. (Fully processed transactions will show as ACTIVE on your payment history page, and can take a few days to fully clear.)
Please note that the default view for the purchase page shows rebilling plans. If you wish to purchase a onetime plan, please click the link that says, "Click here to view our non-renewal plans" under the price grid.