mail in

[Progress]

I have two questions.

If you get a ticket for speeding, you can just mail in the payment.

I haven't seen the usage mail in. Is it common? Does it mean to transer the payment by mail?

But if you play your car radio too loud, you have to come to court to pay the fine.
When you have the radio turned up that loud, it is not safe.

Are the　two loudｓ an adverb or an adjective? I think it is an adverb.

[Anglika]

It is more usual in AmE. In the Uk we would expect "post it".

Yes, they are adverbs.

[Progress]

Thank you very much Anglika