Yes, your idea is feasible with the Square APIs. You can use the Orders API to watch for online orders and check each of the line items for fulfillment. You can also use the UpdateOrder endpoint to change the fulfillment state to PREPARED and COMPLETED.
For issuing refunds, you can use the Refunds API. When you scan the exact price embedded barcode and calculate the overage, you can create a refund for the difference. This will refund the amount back to the customer’s card.
Remember to handle any errors or edge cases that might occur during this process, such as failed refunds or issues with updating the order status. Also, ensure that your application securely handles sensitive customer information, such as card details, in compliance with PCI standards.
This answer was reviewed by @Bryan-Square.