When you click on an order from the order listing page, Shopify will show you a detailed information regarding that order. Which includes but not limited to:

List of items being purchased
Billing information of the customer
Tracking information
Common Reference Number (order_id) between your store order and Fiuu transaction

In this page your will be able to Cancel or Refund the order so that your customer will receive their money back into their account.

Tracking the transaction at Fiuu Merchant Portal

Make note of your order's Payment ID which can be seen as in the following example:
Head over to https://portal.fiuu.com/index.php and log in using credentials emailed by Fiuu right after you successfully onboarded.
Once logged in, you will see your dashboard.
Click on "Transactions" on the sidebar and click "Main accounts" to show list of your transactions
In this page you can see transactions along with their OrderID which should match with shopify's Payment ID in step 1. For example: rbfCsv4XQBU797wlzKRVpMegI
Click on the numeric value in TranID column. And you will be shown this page for your further reference.