Hi Jacob,
Yes, you can do this in Method. You can set up a recurring work order for your billing installment, with the invoice type of "To be Invoiced" and occur on a monthly basis. You may want to assign this work order to someone who takes care of billing or who would be taking care of the invoicing, as opposed to someone who goes out on the field.
Next, you would then schedule services that are included in the contract price as recurring work order with the invoice type of "Do not Invoice"
At the end/ beginning of the month invoice all work orders as you normally would. Within Field Services you can set your filter to show Work Orders within the date range and select multiple ones to be invoiced at the same time. The great thing about this is that your billing installment prices will be included since they have the invoice type "To be invoiced". On the contrary, services will not get invoiced since they have the Invoice type of "Do Not Invoice"
Hope this helps.