checking valid purchase amount from config

pass/routes/key.js

@@ -164,7 +164,7 @@ router.use(
     .optional({ checkFalsy: true })
     .isInt()
     .toInt()
-    .isIn([300, 600, 900, 1200, 1800, 3600]),
+    .isIn(config.get("price.purchasable")),
   (req, res, next) => {
     // Input Validation
     const errors = validationResult(req);