Apologies in advance for a policy, rather than a programming question, but given the paucity of information available online I hope I can be forgiven for asking it here.
If I understand your question, you are offering "another month of this service" as a subscription item. This would be a common model for subscription-based games, etc. Probably your best indication of whether or not it is allowed is to look for other apps that do it.
In addition, don't be afraid to contact Apple! They're not monsters, and they do answer questions! You can email tech support or, if that doesn't get you a helpful response, ask to talk with one of Apple's iOS biz-dev managers. They have people whose job it is to help you make your business succeed. If they don't know the answer, they'll find it; if the answer is "no", they'll help you figure out ways to do something similar. They're quite friendly & knowledgable, like a genius bar dedicated to the iOS app business.
Let us know what you learn!