On two occasions, I returnt items and they refunded me immediately.
The first one, they sent a reminder saying it hadn't been received and if not received by a specific date, the refund would be recovered.
I had proof of the return so ignored it thinking there might be a delay in their system.
I then received a second reminder and when I 'phoned them, it turnt out the item had been received at their end shortly after my returning it.
They apologized, said it would be sorted and not to worry about it but I received another reminder because no-one had updated their system.
The next item I ordered had to be returnt and the same thing happened. It was sitting at their returns site but the system hadn't been updated so a reminder e-mail was sent to me.
I was not a happy bunny and they gave me a £10 credit but that can be used only for items selt by Amazon so it's not as good as it sounds.
If Royal Mail is saying it's not been delivered yet, it might be delayed by the strike action but give Amazon a ring and see what they say.
They should be able to track it at their end and see it's on its way so they might refund you on that basis.