Learn Italian: Paghiamo la sanzione entro la scadenza. - We pay the penalty by the deadline.

Entro is the specific preposition to use when you mean 'no later than' a certain time. It sets a strict deadline. While per can sometimes be used for deadlines (like per domani meaning 'for tomorrow'), entro is the clearest way to say 'within' or 'by' a specific time limit like la scadenza.

Is scadenza only used for paying bills or penalties?

No, scadenza is a very common word for any kind of expiration or due date. For example, you will see data di scadenza on food packaging to mean 'expiration date', and you can also use it for a project deadline at work or school.

La sanzione ends in -e. Is there an easy way to remember that it is feminine?

Yes! A very reliable rule in Italian is that nouns ending in -zione are almost always feminine. You have likely seen this pattern with earlier vocabulary words like la stazione (the station) or la colazione (breakfast).

Since the deadline hasn't happened yet, why is the present tense (paghiamo) used instead of the future tense?

Just like in English, Italian often uses the present tense to state a general rule, a firm intention, or a scheduled future action. You could absolutely use the future tense here (pagheremo meaning 'we will pay'), but the present tense makes it sound like a standard, guaranteed procedure.