Once Scalapay has approved an order, the details and value of the order cannot be changed. You may issue partial or total refunds in case you no longer have the items sold or to meet the customer’s requests for any reason.

You can find more information here:
https://scalapay.zendesk.com/hc/en-gb/articles/360041903113-How-do-I-issue-a-refund-

Scalapay does not allow you to add items to an order already placed.