how to renew domain name registration

Question

When my hosting plan and domain name expires a new invoice is generated but the new invoice has only the hosting plan to renew inside without the domain name even though it should have expired too by date. Only the hosting plan gets suspended. Can anyone tell me if they have this problem and how can I renew my domain name registration. At least tell me if you can renew your domain name from the new generated invoice without a problem... because then I'll know the problem is from my WSP..  Thanks in advance..