diff --git a/opsr/backend/activity-log.ts b/opsr/backend/activity-log.ts
new file mode 100644
index 00000000..79e6948f
--- /dev/null
+++ b/opsr/backend/activity-log.ts
@@ -0,0 +1,60 @@
+// BE-18: User activity log endpoint
+
+type ActivityAction =
+  | "LOGIN"
+  | "LOGOUT"
+  | "PASSWORD_CHANGE"
+  | "SHIPMENT_STATE_CHANGE";
+
+interface ActivityLog {
+  id: string;
+  userId: string;
+  action: ActivityAction;
+  ipAddress: string;
+  userAgent: string;
+  createdAt: Date;
+}
+
+interface PaginationOptions {
+  page: number;
+  pageSize: number;
+}
+
+const logs: ActivityLog[] = [];
+
+function generateId(): string {
+  return Math.random().toString(36).slice(2, 10);
+}
+
+export function logActivity(
+  userId: string,
+  action: ActivityAction,
+  ipAddress: string,
+  userAgent: string
+): ActivityLog {
+  const entry: ActivityLog = {
+    id: generateId(),
+    userId,
+    action,
+    ipAddress,
+    userAgent,
+    createdAt: new Date(),
+  };
+  logs.push(entry);
+  return entry;
+}
+
+export function getUserActivity(
+  userId: string,
+  { page, pageSize }: PaginationOptions
+): { data: ActivityLog[]; total: number } {
+  const userLogs = logs
+    .filter((l) => l.userId === userId)
+    .sort((a, b) => b.createdAt.getTime() - a.createdAt.getTime());
+
+  const start = (page - 1) * pageSize;
+  return {
+    data: userLogs.slice(start, start + pageSize),
+    total: userLogs.length,
+  };
+}
diff --git a/opsr/backend/announcement.ts b/opsr/backend/announcement.ts
new file mode 100644
index 00000000..b642eca5
--- /dev/null
+++ b/opsr/backend/announcement.ts
@@ -0,0 +1,64 @@
+// BE-17: Platform-wide announcement system for admins
+
+interface Announcement {
+  id: string;
+  title: string;
+  body: string;
+  isActive: boolean;
+  expiresAt: Date | null;
+  createdBy: string;
+  createdAt: Date;
+}
+
+const store: Announcement[] = [];
+
+function generateId(): string {
+  return Math.random().toString(36).slice(2, 10);
+}
+
+export function createAnnouncement(
+  title: string,
+  body: string,
+  createdBy: string,
+  expiresAt?: Date
+): Announcement {
+  const announcement: Announcement = {
+    id: generateId(),
+    title,
+    body,
+    isActive: true,
+    expiresAt: expiresAt ?? null,
+    createdBy,
+    createdAt: new Date(),
+  };
+  store.push(announcement);
+  return announcement;
+}
+
+export function getActiveAnnouncements(): Announcement[] {
+  const now = new Date();
+  return store.filter(
+    (a) => a.isActive && (a.expiresAt === null || a.expiresAt > now)
+  );
+}
+
+export function updateAnnouncement(
+  id: string,
+  patch: Partial<Pick<Announcement, "title" | "body" | "isActive" | "expiresAt">>
+): Announcement | null {
+  const item = store.find((a) => a.id === id);
+  if (!item) return null;
+  Object.assign(item, patch);
+  return item;
+}
+
+export function deleteAnnouncement(id: string): boolean {
+  const idx = store.findIndex((a) => a.id === id);
+  if (idx === -1) return false;
+  store.splice(idx, 1);
+  return true;
+}
+
+export function getAllAnnouncements(): Announcement[] {
+  return [...store];
+}
diff --git a/opsr/backend/referral.ts b/opsr/backend/referral.ts
new file mode 100644
index 00000000..32bd594e
--- /dev/null
+++ b/opsr/backend/referral.ts
@@ -0,0 +1,53 @@
+// BE-19: Referral code system for user registration
+
+interface User {
+  id: string;
+  email: string;
+  referralCode: string;
+  referredById: string | null;
+}
+
+const users: User[] = [];
+
+function generateReferralCode(): string {
+  return Math.random().toString(36).slice(2, 8).toUpperCase();
+}
+
+function generateId(): string {
+  return Math.random().toString(36).slice(2, 10);
+}
+
+export function registerUser(
+  email: string,
+  referralCode?: string
+): User | { error: string } {
+  const referrer = referralCode
+    ? users.find((u) => u.referralCode === referralCode)
+    : null;
+
+  if (referralCode && !referrer) {
+    return { error: "Invalid referral code" };
+  }
+
+  const newUser: User = {
+    id: generateId(),
+    email,
+    referralCode: generateReferralCode(),
+    referredById: referrer?.id ?? null,
+  };
+
+  users.push(newUser);
+  return newUser;
+}
+
+export function getUserReferrals(userId: string): User[] {
+  return users.filter((u) => u.referredById === userId);
+}
+
+export function getUserByReferralCode(code: string): User | undefined {
+  return users.find((u) => u.referralCode === code);
+}
+
+export function getAllUsers(): User[] {
+  return [...users];
+}
diff --git a/opsr/backend/shipment-cost-calculator.ts b/opsr/backend/shipment-cost-calculator.ts
new file mode 100644
index 00000000..f6dc361c
--- /dev/null
+++ b/opsr/backend/shipment-cost-calculator.ts
@@ -0,0 +1,58 @@
+// BE-20: Shipment cost calculator endpoint
+
+type CargoType = "standard" | "fragile" | "hazardous" | "perishable";
+
+interface CostInput {
+  origin: string;
+  destination: string;
+  weightKg: number;
+  volumeCbm: number;
+  cargoType: CargoType;
+}
+
+interface CostBreakdown {
+  baseRate: number;
+  weightSurcharge: number;
+  volumeSurcharge: number;
+  cargoSurcharge: number;
+  total: number;
+}
+
+const BASE_RATE = 50;
+const WEIGHT_RATE = 1.2;   // per kg
+const VOLUME_RATE = 8;     // per cbm
+
+const CARGO_MULTIPLIERS: Record<CargoType, number> = {
+  standard: 1.0,
+  fragile: 1.3,
+  hazardous: 1.8,
+  perishable: 1.5,
+};
+
+// Naive distance proxy: hash origin+destination into a 100–2000 km range
+function estimateDistance(origin: string, destination: string): number {
+  const hash = [...(origin + destination)].reduce(
+    (acc, c) => acc + c.charCodeAt(0),
+    0
+  );
+  return 100 + (hash % 1900);
+}
+
+export function calculateShipmentCost(input: CostInput): CostBreakdown {
+  const { origin, destination, weightKg, volumeCbm, cargoType } = input;
+
+  const distanceFactor = estimateDistance(origin, destination) / 1000;
+  const baseRate = BASE_RATE * distanceFactor;
+  const weightSurcharge = weightKg * WEIGHT_RATE * distanceFactor;
+  const volumeSurcharge = volumeCbm * VOLUME_RATE * distanceFactor;
+  const subtotal = baseRate + weightSurcharge + volumeSurcharge;
+  const cargoSurcharge = subtotal * (CARGO_MULTIPLIERS[cargoType] - 1);
+
+  return {
+    baseRate: +baseRate.toFixed(2),
+    weightSurcharge: +weightSurcharge.toFixed(2),
+    volumeSurcharge: +volumeSurcharge.toFixed(2),
+    cargoSurcharge: +cargoSurcharge.toFixed(2),
+    total: +(subtotal + cargoSurcharge).toFixed(2),
+  };
+}