diff --git a/editor.planx.uk/src/@planx/components/Pay/Public/FeeBreakdown/utils.test.ts b/editor.planx.uk/src/@planx/components/Pay/Public/FeeBreakdown/utils.test.ts
new file mode 100644
index 0000000000..25e52596b9
--- /dev/null
+++ b/editor.planx.uk/src/@planx/components/Pay/Public/FeeBreakdown/utils.test.ts
@@ -0,0 +1,71 @@
+import { PassportFeeFields } from "./types";
+import { calculateReduction, toFeeBreakdown, toNumber } from "./utils";
+
+describe("toNumber() helper function", () => {
+  it("outputs a number when passed a number", () => {
+    const input = 12;
+    const output = toNumber(input);
+
+    expect(output).toEqual(input);
+  });
+
+  it("outputs a number when passed a number tuple", () => {
+    const input: [number] = [12];
+    const output = toNumber(input);
+
+    expect(output).toEqual(12);
+  });
+});
+
+describe("calculateReduction() helper function", () => {
+  it("correctly outputs the reduction when a calculated value is provided", () => {
+    const input: PassportFeeFields = {
+      "application.fee.calculated": 100,
+      "application.fee.payable": 50,
+      "application.fee.payable.vat": 0,
+    };
+    const reduction = calculateReduction(input);
+
+    expect(reduction).toEqual(50);
+  });
+
+  it("defaults to 0 when calculated is 0", () => {
+    const input: PassportFeeFields = {
+      "application.fee.calculated": 0,
+      "application.fee.payable": 100,
+      "application.fee.payable.vat": 0,
+    };
+    const reduction = calculateReduction(input);
+
+    expect(reduction).toEqual(0);
+  });
+});
+
+describe("toFeeBreakdown() helper function", () => {
+  it("correctly maps fields", () => {
+    const input: PassportFeeFields = {
+      "application.fee.calculated": 100,
+      "application.fee.payable": 50,
+      "application.fee.payable.vat": 10,
+    };
+
+    const output = toFeeBreakdown(input);
+
+    expect(output.applicationFee).toEqual(input["application.fee.calculated"]);
+    expect(output.total).toEqual(input["application.fee.payable"]);
+    expect(output.vat).toEqual(input["application.fee.payable.vat"]);
+    expect(output.reduction).toEqual(50);
+  });
+
+  it("sets applicationFee to payable amount if no calculated value is provided", () => {
+    const input: PassportFeeFields = {
+      "application.fee.calculated": 0,
+      "application.fee.payable": 50,
+      "application.fee.payable.vat": 10,
+    };
+
+    const output = toFeeBreakdown(input);
+
+    expect(output.applicationFee).toEqual(input["application.fee.payable"]);
+  });
+});
diff --git a/editor.planx.uk/src/@planx/components/Pay/Public/FeeBreakdown/utils.ts b/editor.planx.uk/src/@planx/components/Pay/Public/FeeBreakdown/utils.ts
index 90c76d260d..cd8b41c859 100644
--- a/editor.planx.uk/src/@planx/components/Pay/Public/FeeBreakdown/utils.ts
+++ b/editor.planx.uk/src/@planx/components/Pay/Public/FeeBreakdown/utils.ts
@@ -2,15 +2,15 @@ import { z } from "zod";
 
 import { FeeBreakdown, PassportFeeFields } from "./types";
 
-const toNumber = (input: number | number[]) =>
-  Array.isArray(input) ? Number(input[0]) : input;
+export const toNumber = (input: number | [number]) =>
+  Array.isArray(input) ? input[0] : input;
 
-const calculateReduction = (data: PassportFeeFields) =>
+export const calculateReduction = (data: PassportFeeFields) =>
   data["application.fee.calculated"]
     ? data["application.fee.calculated"] - data["application.fee.payable"]
     : 0;
 
-const toFeeBreakdown = (data: PassportFeeFields): FeeBreakdown => ({
+export const toFeeBreakdown = (data: PassportFeeFields): FeeBreakdown => ({
   applicationFee:
     data["application.fee.calculated"] || data["application.fee.payable"],
   total: data["application.fee.payable"],
@@ -19,8 +19,8 @@ const toFeeBreakdown = (data: PassportFeeFields): FeeBreakdown => ({
 });
 
 export const createPassportSchema = () => {
-  const questionSchema = z.number();
-  const setValueSchema = z.array(z.coerce.number());
+  const questionSchema = z.number().positive();
+  const setValueSchema = z.tuple([z.coerce.number().positive()]);
   const feeSchema = z
     .union([questionSchema, setValueSchema])
     .transform(toNumber);