Hanlon's razor "Never attribute to malice that which is adequately explained by stupidity"
I don't believe for a second that Planet X has a policy of stiffing it's customers by failing to process returns. That would screw their reputation so fast it wouldn't be worth it. They make better money by being honest.
Or that someone in their company is stealing credit from customer accounts. That would be a terrible way of embezzling. 1) Customers would raise hell, the company would notice 2) it is still just credit at Planet X, you need another step or more to turn it into cash, which would all leave a trail.
Or that someone in the company is deliberately not processing returns. Again, you have a queue of angry customers and your reward is a pile of lightly worn discount bicycle shorts?
So Occam and Hanlon lead us to error rather than theft. Either Accy has misremembered, someone made a mistake on the returns, or the credit was accidentally (human or software) applied to another customers purchase, or the credit was applied to accy's purchase rather than from his Lloyd's account.