How many host names do you have already created? Your DNS Pro/Remote Access service allow for up to 30 host names per service pack. If you already have 30, then you would need to purchase an additional service pack in order to add the new one.
You can remove the service from your cart by clicking on your cart via account.dyn.com and then "remove service" from it. It will not have an impact on your existing services you have paid for.
Oracle + Dyn