Learn Italian: Signori, paghino la tassa allo sportello. - Gentlemen, pay the fee at the counter.

This is the formal plural imperative (used for the formal "you all", Loro). For -are verbs like pagare, the formal plural command ends in -ino. If it were a regular statement ("they pay"), you would use the present tense pagano.

Where did the "h" in paghino come from?

The verb is pagare. When we change the ending to start with an "i" or "e", we have to add an "h" to keep the hard "g" sound. Without the "h", it would be pronounced with a soft "j" sound.

Why is it allo sportello and not just al sportello?

Allo is the combination of the preposition a (at/to) and the definite article lo (the). We must use lo instead of il because sportello starts with an "s" followed by another consonant (s + p).

If I were telling a group of friends to pay, would I still use paghino?

No, paghino is only for speaking to a group formally, like these Signori. If you were talking to friends, you would use the informal plural (voi) command, which is pagate.