Changes based on review

required php-api-library min 1.5.2
Using storeId for retrieving wsUsername and wsPassword

Helper/Data.php

@@ -1174,8 +1174,8 @@ class Data extends AbstractHelper
     public function initializeAdyenClient($storeId = null)
 	{
 		// initialize client
-		$webserviceUsername = $this->getWsUsername();
-		$webservicePassword = $this->getWsPassword();
+		$webserviceUsername = $this->getWsUsername($storeId);
+		$webservicePassword = $this->getWsPassword($storeId);
 
 		$client = new \Adyen\Client();
 		$client->setApplicationName("Magento 2 plugin");

composer.json

@@ -14,7 +14,7 @@
     }
   ],
   "require": {
-    "adyen/php-api-library": ">1.5.0",
+    "adyen/php-api-library": ">=1.5.2",
     "magento/framework": ">=100.1.0"
   },
   "autoload": {