diff --git a/contracts/src/Chainvoice.sol b/contracts/src/Chainvoice.sol
index 042397b4..bdf637bd 100644
--- a/contracts/src/Chainvoice.sol
+++ b/contracts/src/Chainvoice.sol
@@ -100,6 +100,27 @@ contract Chainvoice {
         _entered = false;
     }
 
+    modifier validInvoice(uint256 invoiceId) {
+        require(invoiceId < invoices.length, "Invalid invoice ID");
+        _;
+    }
+
+    modifier onlyCreator(uint256 invoiceId) {
+        require(msg.sender == invoices[invoiceId].from, "Only invoice creator can cancel");
+        _;
+    }
+
+    modifier onlyPayer(uint256 invoiceId) {
+        require(msg.sender == invoices[invoiceId].to, "Not authorized");
+        _;
+    }
+
+    modifier invoiceActive(uint256 invoiceId) {
+        require(!invoices[invoiceId].isPaid, "Already paid");
+        require(!invoices[invoiceId].isCancelled, "Invoice is cancelled");
+        _;
+    }
+
     // Constants
     uint256 public constant MAX_BATCH = 50;
 
@@ -217,12 +238,8 @@ contract Chainvoice {
     }
 
     // ========== Cancel single invoice ==========
-    function cancelInvoice(uint256 invoiceId) external {
-        if (invoiceId >= invoices.length) revert InvalidInvoiceId();
+    function cancelInvoice(uint256 invoiceId) external validInvoice(invoiceId) onlyCreator(invoiceId) invoiceActive(invoiceId) {
         InvoiceDetails storage invoice = invoices[invoiceId];
-
-        if (msg.sender != invoice.from) revert NotInvoiceCreator();
-        if (invoice.isPaid || invoice.isCancelled) revert InvoiceNotCancellable();
         
         invoice.isCancelled = true;
 
@@ -235,14 +252,9 @@ contract Chainvoice {
     }
 
     // ========== Pay single invoice ==========
-    function payInvoice(uint256 invoiceId) external payable nonReentrant {
-        if (invoiceId >= invoices.length) revert InvalidInvoiceId();
+    function payInvoice(uint256 invoiceId) external payable nonReentrant validInvoice(invoiceId) onlyPayer(invoiceId) invoiceActive(invoiceId) {
         InvoiceDetails storage invoice = invoices[invoiceId];
         
-        if (msg.sender != invoice.to) revert NotAuthorizedPayer();
-        if (invoice.isPaid) revert InvoiceAlreadyPaid();
-        if (invoice.isCancelled) revert InvoiceCancelledError();
-
         // Effects first for CEI (mark paid, bump fees), then interactions
         invoice.isPaid = true;
         if (invoice.tokenAddress == address(0)) {
@@ -252,10 +264,11 @@ contract Chainvoice {
             (bool sent, ) = payable(invoice.from).call{value: invoice.amountDue}("");
             if (!sent) revert NativeTransferFailed();
         } else {
-            if (msg.value != fee) revert FeeMustBeNative();
-            if (IERC20(invoice.tokenAddress).allowance(msg.sender, address(this)) < invoice.amountDue) {
-                revert InsufficientAllowance();
-            }
+            require(msg.value == fee, "Must pay fee in native token");
+            require(
+                IERC20(invoice.tokenAddress).allowance(msg.sender, address(this)) >= invoice.amountDue,
+                "Insufficient allowance"
+            );
             accumulatedFees += fee;
 
             bool transferSuccess = IERC20(invoice.tokenAddress).transferFrom(
@@ -274,7 +287,6 @@ contract Chainvoice {
             invoice.tokenAddress
         );
     }
-
     // ========== Batch pay (all-or-nothing) ==========
     function payInvoicesBatch(uint256[] calldata invoiceIds) external payable nonReentrant {
         uint256 n = invoiceIds.length;
@@ -351,6 +363,7 @@ contract Chainvoice {
     )
         external
         view
+        validInvoice(invoiceId)
         returns (bool canPay, uint256 payerBalance, uint256 allowanceAmount)
     {
         if (invoiceId >= invoices.length) revert InvalidInvoiceId();
@@ -394,8 +407,7 @@ contract Chainvoice {
         return result;
     }
 
-    function getInvoice(uint256 invoiceId) external view returns (InvoiceDetails memory) {
-        if (invoiceId >= invoices.length) revert InvalidInvoiceId();
+    function getInvoice(uint256 invoiceId) external view validInvoice(invoiceId) returns (InvoiceDetails memory) {
         return invoices[invoiceId];
     }