Fix online payment surcharge — input and validation based on total

The payment amount input was capped at BalanceDue (e.g. $5.00) but the
customer was being charged TotalWithSurcharge (e.g. $5.15), causing
validation to reject any attempt to pay the correct total amount.

Input now defaults to and accepts up to TotalWithSurcharge. On submit,
the JS back-calculates the base amount before sending to the server so
the server-side surcharge addition produces the same PaymentIntent total
that the Stripe Elements were initialized with — eliminating the
amount-mismatch error from Stripe on confirmation.

Also calls elements.update() when the amount changes so partial payments
don't cause an Elements/PaymentIntent amount mismatch.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/PowderCoating.Web/Views/Payment/Index.cshtml

@@ -75,11 +75,11 @@
                 <div class="input-group mb-1">
                     <span class="input-group-text">$</span>
                     <input type="number" id="paymentAmount" class="form-control"
-                           value="@Model.BalanceDue.ToString("F2")"
+                           value="@Model.TotalWithSurcharge.ToString("F2")"
-                           min="0.01" max="@Model.BalanceDue.ToString("F2")" step="0.01"
+                           min="0.01" max="@Model.TotalWithSurcharge.ToString("F2")" step="0.01"
                            oninput="updateSurcharge()" />
                 </div>
-                <div class="form-text">Max: @Model.BalanceDue.ToString("C")</div>
+                <div class="form-text">Max: @Model.TotalWithSurcharge.ToString("C")</div>
             </div>
         </div>
 
@@ -116,7 +116,7 @@
     const STRIPE_PK  = '@Model.StripePublishableKey';
     const ACCOUNT_ID = '@Model.StripeAccountId';
     const TOKEN      = '@Model.Token';
-    const MAX_AMOUNT = @Model.BalanceDue.ToString("F2");
+    const MAX_TOTAL  = @Model.TotalWithSurcharge.ToString("F2");
     const SURCHARGE_TYPE  = '@Model.SurchargeType';
     const SURCHARGE_VALUE = @Model.SurchargeValue.ToString("F4");
     const SUCCESS_URL = `/pay/${TOKEN}/success`;
@@ -125,9 +125,8 @@
     let elements, paymentElement;
 
     async function initStripe() {
-        const amount = parseFloat(document.getElementById('paymentAmount').value) || MAX_AMOUNT;
+        // Input value is the total the customer pays (including fee)
-        const surcharge = calcSurcharge(amount);
+        const total = parseFloat(document.getElementById('paymentAmount').value) || MAX_TOTAL;
-        const total = Math.round((amount + surcharge) * 100) / 100;
 
         elements = stripe.elements({
             mode: 'payment',
@@ -141,12 +140,15 @@
         paymentElement.mount('#payment-element');
     }
 
-    function calcSurcharge(amount) {
+    // Back-calculates the base invoice amount from a total that includes the fee.
+    // Server receives the base and adds surcharge on top — both paths arrive at the same
+    // PaymentIntent amount so Stripe.js never sees an amount mismatch.
+    function calcBaseFromTotal(total) {
         if (SURCHARGE_TYPE === 'Percent')
-            return Math.round(amount * (SURCHARGE_VALUE / 100) * 100) / 100;
+            return Math.round(total / (1 + SURCHARGE_VALUE / 100) * 100) / 100;
         if (SURCHARGE_TYPE === 'Flat')
-            return SURCHARGE_VALUE;
+            return Math.max(0, Math.round((total - SURCHARGE_VALUE) * 100) / 100);
-        return 0;
+        return total;
     }
 
     function formatCurrency(val) {
@@ -154,9 +156,9 @@
     }
 
     function updateSurcharge() {
-        const amount = Math.min(parseFloat(document.getElementById('paymentAmount').value) || 0, MAX_AMOUNT);
+        const total = Math.min(parseFloat(document.getElementById('paymentAmount').value) || 0, MAX_TOTAL);
-        const surcharge = calcSurcharge(amount);
+        const base = calcBaseFromTotal(total);
-        const total = amount + surcharge;
+        const surcharge = Math.round((total - base) * 100) / 100;
 
         const surchargeEl = document.getElementById('surchargeDisplay');
         const totalEl = document.getElementById('totalWithSurchargeDisplay');
@@ -164,7 +166,10 @@
 
         if (surchargeEl) surchargeEl.textContent = formatCurrency(surcharge);
         if (totalEl) totalEl.textContent = formatCurrency(total);
-        if (btnAmtEl) btnAmtEl.textContent = surcharge > 0 ? formatCurrency(total) : formatCurrency(amount);
+        if (btnAmtEl) btnAmtEl.textContent = formatCurrency(total);
+
+        // Keep Elements in sync so confirmPayment amount matches the PaymentIntent
+        if (elements) elements.update({ amount: Math.round(total * 100) });
     }
 
     async function submitPayment() {
@@ -172,13 +177,21 @@
         const msgEl = document.getElementById('payment-message');
         const amountInput = document.getElementById('paymentAmount');
 
-        const amount = parseFloat(amountInput.value);
+        const total = parseFloat(amountInput.value);
-        if (!amount || amount <= 0 || amount > MAX_AMOUNT) {
+        if (!total || total <= 0 || total > MAX_TOTAL) {
             msgEl.textContent = 'Please enter a valid payment amount.';
             msgEl.style.display = '';
             return;
         }
 
+        // Back-calculate base; server adds surcharge on top as usual
+        const base = calcBaseFromTotal(total);
+        if (base <= 0) {
+            msgEl.textContent = 'Payment amount must exceed the convenience fee.';
+            msgEl.style.display = '';
+            return;
+        }
+
         btn.disabled = true;
         document.getElementById('submitText').style.display = 'none';
         document.getElementById('submitSpinner').style.display = '';
@@ -195,13 +208,13 @@
             return;
         }
 
-        // Create PaymentIntent on server
+        // Create PaymentIntent on server — send base amount (server adds surcharge)
         let clientSecret;
         try {
             const resp = await fetch(`/pay/${TOKEN}/intent`, {
                 method: 'POST',
                 headers: { 'Content-Type': 'application/json' },
-                body: JSON.stringify({ amount })
+                body: JSON.stringify({ amount: base })
             });
             const data = await resp.json();
             if (!resp.ok) {