they have decided to use the post office to send letters 同义句

they decied to use the post office to send letters
they have decied to send letters by post office.

Sending letters by the post office has been gained their approve.
或者Because of their request ,the post offfice will play a role in sending their letters.

It have been decided by them to use the post office to send letters.
原意不变换个句式 So easy~~

they have decided to send letters from the post office

they have decided to use the post office for sending letters