is there a way to use service agreements for routine maintenance contracts?
What do you mean? What accounting question are you asking?
@kens if you are asking can a service agreement be posted into Manager, so that it can produce periodical (monthly) invoices for the term of the contract. The short answer is no, but after entering your first invoice for a client you can clone that invoice for the future periods and edit any variables.
If you are trying to invoice the same customer for the same thing month after month, you can do that with a recurring sales invoice. Read the Guides starting at http://guides.manager.io/businesses/settings/recurring-sales-invoices.