Learn Italian: Attenda il timbro su questa lettera. - Wait for the stamp on this letter.

Attenda is the formal imperative of attendere (to wait). When giving polite commands to someone using Lei, verbs that end in -ere or -ire take an -a ending. If you were speaking informally to a friend, you would say attendi.

The English translation says 'Wait for', so why isn't there a word for 'for' after attenda?

The Italian verb attendere already means 'to wait for.' It connects directly to the thing you are waiting for, so you don't need to add a preposition like per. You just say attenda il timbro.

Is a timbro the kind of stamp you buy to mail a letter?

No, a timbro is an ink stamp, seal, or postmark used to mark official documents. If you are talking about the sticky postage stamp you buy to mail a letter, you would use the word francobollo.

Why do we use questa instead of questo here?

The word for 'this' has to match the noun it describes. Since lettera is a feminine singular noun, questo changes to its feminine singular form, questa.