這句子是甚麼文法？很奇怪唷

He shouldn't use what, but which, which refers to the envelop

What he meant is: A pre-addressed and stamped envelope which to mail your membership application to our club is enclosed in this letter.

But it is a better way to express: Please mail your membership application to our club with the enclosed pre-addressed, stamped envelope.

The grammar of this sentence is correct, but poorly written.

Basically, the sentence should read like this:

A pre-addressed, stamped envelope, what to mail your application for membership to our club, is enclosed with this letter.