I would ignore Yodel telling you the buyer smashed it with a hammer. I doubt anyone could prove it, so you would need to refund the buyer (as you have done already).
In terms of refunds - the process is that the buyer opens a case stating the item was damaged on arrival and you accept the case and the refund is provided automatically.
It is better to do that than just refund the buyer using another option/method. The case will show you have refunded. If the buyer claims they've not received it, tell them to discuss it with ebay.
I think it can take a few days after you accept the refund for the money to be back with the buyer.
Here's a Royal Mail video on packaging fragile items.
https://www.youtube.com/watch?v=4FJEC98_uhU
I would suggest a bigger and stronger box than she uses - preferably two boxes.
Also write “FRAGILE HANDLE WITH CARE” in capital letters on the cover or envelope above the address.
Some people have different views about how well different carriers look after packages; and different services from the same carrier can be treated differently.
E.g. Royal Mail Special Delivery should be passed to someone, not just left somewhere.
I suggest you look at the carrier's insurance/compensation policy and what it says about fragile items. It may be worth videoing the packaging used to show that it matches their guidelines, if you want to try and claim compensation from the carrier (outside ebay).
Depending on what you experience, the value of the items to you, how many you have etc. you may want to consider selling them somewhere else e.g. not posting them. Or you can sell on ebay with Collection Only for example.
I doubt you can literally stop it happening. Hopefully you can reduce the chances of it happening.
