Flow: service billing (transport, hostel, meals)

Transport, boarding and catering are separate apps, but a family never gets three bills. Each service posts its charge onto the student's one fee assignment through a bridge — with no double entry.

Service billing — each service's bridge writes one ep.fee.assignment.line; Fees recomputes the total.

Mechanics per service

Service	Source object	Method	Fee category	Amount field
Transport	`ep.transport.assignment`	`_create_fee_line()`	`ep_fees.fee_cat_transport`	`effective_fee`
Hostel	`ep.hostel.allocation`	`_create_fee_lines()`	boarding category	room / term fee
Meals	`ep.meals.subscription`	`_create_fee_line()`	meals category	plan price
Library	overdue loan	fine posting	fines	fine amount

Recompute, never duplicate

Each bridge calls _compute_totals() and _compute_paid() on the ep.fee.assignment after writing the line, so the invoice and the family balance are instantly correct. Removing the service (_remove_fee_line()) unlinks the line and recomputes — no orphan charges.

Edge cases

Staff who commute: staff_commuter assignments are skipped — staff ride free, no fee line.
Mid-year join: if there is no active ep.fee.assignment yet, the bridge posts a note and creates the line at enrolment instead.
Pausing a service: pausing the assignment removes the line; the family stops being billed from that point.

No manual entry — Never type transport/hostel/meal charges into Fees by hand — the bridge owns them, so the invoice is always complete and reversible.