It's likely because the purchase set up a monthly subscription. You can cancel an active subscription by following these steps:
1. From the RS homepage, click Account in the upper right-hand corner. 2. Log in using your account's details. 3. In the Membership panel on the right-hand side, click Manage your subscriptions. 4. Click Manage by account. 5. Log in using your account's details. 6. Here, you should see the subscription listed; simply click Cancel on the far right side.