我的信一定是在邮寄过程中弄丢了。

My letter must have got lost in the ___post.

如果你不想亲自送过去，你可以邮寄。

If you don't want to take it there, you can just send it by ___post.

所以你必须邮寄它。

So you have to send it by ___post.