You have no claim against Amazon but do have a claim against the buyer.
You need to issue a letter before action, you need to explain that the Amazon A2Z process is not binding arbitration and that while funds have been reallocated that a debt has now been created which can be discharged by either returning the goods, by sending payment, or by arranging with Amazon to recharge their card.
You need to set a deadline and explain that the next step will be to issue a County Court Claim and that the issue fee for the claim will be added in addition to statutory interest at 8% per year from the date that Amazon reallocated your funds.
In terms of filing your claim, its fairly straightforward, you will need to specify who the parties are and the relationship between the parties. And an explanation of how the debt accrued.
Your fee will be fairly inexpensive for this, if filed online the cost would be £35. The process is very easy, I have sued lots of organisations in the past. Most people will either pay up or return the item. No one wants the risk of their credit rating being destroyed for 6 years, for some a CCJ could even end their career - such as those working in financial services, solicitors, accountants, Police officers.
Worst case scenario is that it end up at hearing but in a case like this you have ample evidence in the form of the order proving formation of contract, your delivery POD which matches the sellers address. It would be open/close and up to the defendant to prove that they did not receive the item. The hearing would be in chambers, in other words a district judges office and the whole process is very informal.
While I am not a lawyer I have been down this route lots of times through both commercial and personal disputes and would be more than happy to help you.