Learn Italian: Qualora Lei non abbia la ricevuta, non possiamo rimborsare il biglietto. - Should you not have the receipt, we cannot refund the ticket.

Qualora means "if", "should", or "in the event that". Because it introduces a hypothetical condition, it strictly requires the subjunctive mood. Since this is about a present or near-future situation, we use the present subjunctive abbia.

How would this sentence change if I used se (if) instead of qualora?

If you use se for a realistic condition like this, you must use the indicative mood rather than the subjunctive: Se Lei non ha la ricevuta.... Using qualora elevates the register, making it sound more formal and perfectly suited for a customer service context.

Why is the pronoun Lei explicitly included and capitalized?

The capitalized Lei is the formal "you". While Italian often drops subject pronouns, it is sometimes included in formal or administrative contexts to be exceptionally polite, clear, and direct.

Why does the main clause use the indicative (possiamo) when the first half uses the subjunctive?

The subjunctive is only triggered inside the dependent clause introduced by qualora. The main clause (non possiamo rimborsare il biglietto) describes a factual, definite consequence of that condition, so it stays in the normal indicative mood.