How to Get Refund on accidental subscription at google play after free trial
Can try the following (though no guarantee that the refund will be successful, but there have been successful cases)
1) call Google HK Customer Service at 800-906-838 during their office hours
2) tell them your problem, They will send you an email to authenticate that the account belongs to you
3) open the email, input your info into the Form. Remember to choose "Chinese Hong Kong" as the language
4) they will call you back in around 1 minutes
5) explain your problem to them
6) they will take a couple of minutes to check your transaction, then another few minutes to see if approval for refund can be granted. They will tell you the result over phone, then through email.
Hope it helps.
Also remember to check setting at google play store to ensure authentication for every purchase is active.