Service Fees can be (1) fixed or consist of (2) costs plus mark-up. Regarding payment deadlines, you should consider whether payment deadlines are appropriate or necessary in an intra-group context.
Costs plus mark-up can be calculated through (a) flexible invoicing and payment terms thus enabling maximum flexibility, (b) payment of actual fees, calculated quarterly or (c) annual budget, on-account invoicing and year-end reconciliation