diff --git a/src/shipping.js b/src/shipping.js
--- a/src/shipping.js
+++ b/src/shipping.js
@@ -1,3 +1,8 @@
-export function shippingFee(orderTotal) {
-  return orderTotal >= 99 ? 0 : 10;
+export function shippingFee(orderTotal, isMember = false) {
+  if (!Number.isFinite(orderTotal) || orderTotal < 0) {
+    throw new RangeError('orderTotal must be a non-negative finite number');
+  }
+
+  const freeShippingThreshold = isMember ? 59 : 99;
+  return orderTotal >= freeShippingThreshold ? 0 : 10;
 }