diff --git a/contracts/BalanceTrackerFixedPriceBase.sol b/contracts/BalanceTrackerFixedPriceBase.sol
index b64f4e8..9bc5c78 100644
--- a/contracts/BalanceTrackerFixedPriceBase.sol
+++ b/contracts/BalanceTrackerFixedPriceBase.sol
@@ -224,7 +224,8 @@ abstract contract BalanceTrackerFixedPriceBase {
 
         // Get mech balance
         uint256 balance = mapMechBalances[mech];
-        if (balance == 0) {
+        // If balance is 1, the marketplace fee is still 1, and thus mech payment will be zero
+        if (balance < 2) {
             revert ZeroValue();
         }
 
diff --git a/contracts/OlasMech.sol b/contracts/OlasMech.sol
index 1497bc6..273b404 100644
--- a/contracts/OlasMech.sol
+++ b/contracts/OlasMech.sol
@@ -223,7 +223,7 @@ abstract contract OlasMech is Mech, IErrorsMech, ImmutableStorage {
     function requestFromMarketplace(address requester, bytes memory data, uint256 requestId) external {
         // Check for marketplace access
         if (msg.sender != mechMarketplace) {
-            revert MarketplaceNotAuthorized(msg.sender);
+            revert MarketplaceOnly(msg.sender, mechMarketplace);
         }
 
         // Perform a request
@@ -236,7 +236,7 @@ abstract contract OlasMech is Mech, IErrorsMech, ImmutableStorage {
     function revokeRequest(uint256 requestId) external {
         // Check for marketplace access
         if (msg.sender != mechMarketplace) {
-            revert MarketplaceNotAuthorized(msg.sender);
+            revert MarketplaceOnly(msg.sender, mechMarketplace);
         }
 
         address requester = mapRequestAddresses[requestId];
diff --git a/contracts/interfaces/IErrorsMech.sol b/contracts/interfaces/IErrorsMech.sol
index e244c82..565aeb8 100644
--- a/contracts/interfaces/IErrorsMech.sol
+++ b/contracts/interfaces/IErrorsMech.sol
@@ -11,9 +11,10 @@ interface IErrorsMech {
     /// @dev The contract is already initialized.
     error AlreadyInitialized();
 
-    /// @dev Mech marketplace is not authorized.
-    /// @param mechMarketplace Mech marketplace address.
-    error MarketplaceNotAuthorized(address mechMarketplace);
+    /// @dev Only `marketplace` has a privilege, but the `sender` was provided.
+    /// @param sender Sender address.
+    /// @param marketplace Required marketplace address.
+    error MarketplaceOnly(address sender, address marketplace);
 
     /// @dev Request Id not found.
     /// @param requestId Request Id.
diff --git a/test/MechFixedPriceNative.js b/test/MechFixedPriceNative.js
index c4feb46..7aa735c 100644
--- a/test/MechFixedPriceNative.js
+++ b/test/MechFixedPriceNative.js
@@ -143,7 +143,7 @@ describe("MechFixedPriceNative", function () {
             // Try to post a request directly to the mech
             await expect(
                 priorityMech.requestFromMarketplace(deployer.address, data, 0)
-            ).to.be.revertedWithCustomError(priorityMech, "MarketplaceNotAuthorized");
+            ).to.be.revertedWithCustomError(priorityMech, "MarketplaceOnly");
 
             // Response time is out of bounds
             await expect(
@@ -224,6 +224,11 @@ describe("MechFixedPriceNative", function () {
                 priorityMech.deliverToMarketplace(requestId, data)
             ).to.be.revertedWithCustomError(priorityMech, "RequestIdNotFound");
 
+            // Try to check and record delivery rate not by marketplace
+            await expect(
+                balanceTrackerFixedPriceNative.checkAndRecordDeliveryRate(deployer.address, 0, "0x")
+            ).to.be.revertedWithCustomError(balanceTrackerFixedPriceNative, "MarketplaceOnly");
+
             // Create a request
             await mechMarketplace.request(data, mechServiceId, requesterServiceId, minResponseTimeout, "0x", {value: maxDeliveryRate});
 
@@ -232,6 +237,11 @@ describe("MechFixedPriceNative", function () {
                 priorityMech.connect(signers[1]).deliverToMarketplace(requestId, data)
             ).to.be.reverted;
 
+            // Try to finalize delivery rate not by marketplace
+            await expect(
+                balanceTrackerFixedPriceNative.finalizeDeliveryRate(priorityMech.address, deployer.address, 0, 0)
+            ).to.be.revertedWithCustomError(balanceTrackerFixedPriceNative, "MarketplaceOnly");
+
             // Get the request status (requested priority)
             status = await mechMarketplace.getRequestStatus(requestId);
             expect(status).to.equal(1);
@@ -302,6 +312,17 @@ describe("MechFixedPriceNative", function () {
             let mechBalance = await balanceTrackerFixedPriceNative.mapMechBalances(priorityMech.address);
             expect(mechBalance).to.equal(maxDeliveryRate);
 
+            // Try to collect fees before any payment processing
+            await expect(
+                balanceTrackerFixedPriceNative.drain()
+            ).to.be.revertedWithCustomError(balanceTrackerFixedPriceNative, "ZeroValue");
+
+            // Try to process payment for mech not by its service multisig
+            await expect(
+                balanceTrackerFixedPriceNative.connect(signers[1]).processPaymentByMultisig(priorityMech.address)
+            ).to.be.revertedWithCustomError(balanceTrackerFixedPriceNative, "UnauthorizedAccount");
+
+
             const balanceBefore = await ethers.provider.getBalance(priorityMech.address);
             // Process payment for mech
             await balanceTrackerFixedPriceNative.processPaymentByMultisig(priorityMech.address);
diff --git a/test/MechMarketplace.js b/test/MechMarketplace.js
index b64661b..a1cc90c 100644
--- a/test/MechMarketplace.js
+++ b/test/MechMarketplace.js
@@ -57,6 +57,10 @@ describe("MechMarketplace", function () {
         const mechMarketplaceProxy = await MechMarketplaceProxy.deploy(mechMarketplace.address, proxyData);
         await mechMarketplaceProxy.deployed();
 
+        // Get implementation
+        const implementation = await mechMarketplaceProxy.getImplementation();
+        expect(implementation).to.equal(mechMarketplace.address);
+
         mechMarketplace = await ethers.getContractAt("MechMarketplace", mechMarketplaceProxy.address);
 
         // Deploy mech factory
@@ -148,7 +152,7 @@ describe("MechMarketplace", function () {
             ).to.be.revertedWithCustomError(mechMarketplace, "ZeroAddress");
 
             // Changing the implementation
-            await mechMarketplace.connect(deployer).changeOwner(mechMarketplace.address);
+            await mechMarketplace.connect(deployer).changeImplementation(mechMarketplace.address);
         });
 
         it("Change marketplace params", async function () {
@@ -183,6 +187,9 @@ describe("MechMarketplace", function () {
             await expect(
                 mechMarketplace.changeMarketplaceParams(10, 10, maxUint96)
             ).to.be.revertedWithCustomError(mechMarketplace, "Overflow");
+
+            // Change params
+            await mechMarketplace.changeMarketplaceParams(fee, minResponseTimeout, maxResponseTimeout);
         });
 
         it("Factories and balance trackers", async function () {