Add a new draft schedule to the catalog.
Effective-from drives which schedule a bill-run picks up. Activating supersedes any prior active schedule for the same (service, region).
Base rate applied before the tier ladder. Use 0 if the schedule is pure tier-driven.
At least one tier is required. Leave Max blank for "unlimited" (top tier).
Optional. Applied to counts above the top tier on a per-unit basis.