I don't think there are any RM services that offer £50 compensation, so probably best to ignore your buyer.
However, if you sent your item using Ebay Simple Delivery, then the item is covered for damage in transit.
If you did use Simple Delivery, the buyer should start a return stating that the item was 'damaged in transit'.
Ebay will compensate them without requiring a return and you will keep the buyer's payment.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)