Andrej's answer is perfect. There are as well other examples: Einen Liter Milch, bitte! Sentences like that, when ordering something in a restaurant or shop, are quite often used.

There is no verb. It is, if you wish, "shorthand" for "Ich möchte bitte eine Fahrkarte zweiter Klasse." But as both travellers and clerks at train stations are often pressed for time the shortened form is acceptable.

bitte is not a verb