You can create a recurring invoice. To do that, create a manual invoice and look for the recurring options. The invoice will be used like a stamp for stamping out invoices going forward. Clients > Select a Client, click the [+] icon in the invoices widget or under the Actions section on the left.
If you did go with a package and you are setting these up for clients yourself, you can set a price override and use the same package. If a price override, create the service using the package (uncheck invoice option), then edit the service and set a price override. You may need to bill the initial month manually (or let Blesta invoice and modify the invoice right away).