My AirPods Max may have liquid damage. How can I ship them to Apple for repair?

my AirPods max got sweat stains on the cups and the headband tried to clean them with water but I think they got liquid damage and they stopped working and it didn't clean them either.

the nearest Apple store is 2 hours from where I live and I am unable to take them there to be repaired.

is there a way to send them to Apple repair in the post how much would it cost and would they be able to replace the cups and headband as well as fixing the sound? I'm in the UK.

[Re-Titled by Moderator]