Why are A-Z claims so unfair???

A customer ordered a T-Shirt from me, it was a Kids Size 7-8 and they paid next day delivery, i posted them the Kids 7-8 and they recieved the t-shirt the next day, customer opens a return stating different to what was ordered, claiming they ordered a size XL. The order definitely shows as a Kids 7-8. The customer even messaged me saying they ''dont know how they ended up ordering a kids 7-8 because they was definitely on the XL or 2XL as they was debating between the 2'' The customer then messaged me a few days later saying they returned the t-shirt and want a full refund, i hadn't recieved the return back at that point and stated all returns through amazon are auto refunded at first scan anyway. a few hours later the customer opens an A-Z claim against me, soon as the claim was opened i just clicked to refund the customer for the t-shirt to save the hassle with it, but did not refund the next day delivery postage costs they paid because it's not my mistake. I wrote a long detailed explanation in the A-Z claim explaining everything yet Amazon have closed the A-Z case today and just taken the next day delivery postage costs from my account to refund the customer. How is this even fair in the slightest, i can only assume this was a customer mistake however it could of also been an error on amazons system as the order went through but that's probably unlikely.

if the customer orders a kids size 7-8 t shirt and pays for next day delivery, and i deliver them a kids 7-8 size t-shirt the very next day. Why on earth would i be responsible to refund the next day delivery postage costs because of a customer or amazons mistake??? I fullfilled the order my end correctly in line with amazons policy.

Amazon don't realise things like this cripple small business's, let me break it down for you, it costs me £6 to make the t-shirt, it costs me £10 to pay for next day delivery, that's £16, the customer opened a returned which i did recieve in the end, which means ive paid nearly £4 return postage, that's £20 in costs were on, the customer then recieves a refund for the next day delivery postage that i've already paid for, that's £30 im now out of pocket on a t-shirt i was originally only making £4ish on, i mean ok i've had the t-shirt returned back to me but i don't re-sell returns i always make fresh t-shirts, the way most t-shirts are returned to me are unusable anyway. It's an absolute joke, seller support told me to appeal and once ive appealed the outcome will be final either way. Which i know i won't win the appeal because i wrote everything in detail before the A-Z was granted to the customer anyway. Losing my wits with this platform i really am.