。
She sent the letter by airmail.
Literal
She [topic-は] letter [object-を] airmail [by-で] sent.
航空便 (こうくうびん) means airmail — 航空 (aviation) + 便 (mail/service). The instrumental で marks the method of sending. This is the formal term; in casual speech, you might hear エアメール (from English 'airmail'). 送る (おくる) is the general verb for sending.