Las is the direct object pronoun that matches the third person plural. A direct object pronoun replaces a direct object, that is a noun that directly receives the action of a verb in a sentence. Moreover, direct object pronouns in Spanish agree in number and gender with the nouns they replace. So the direct object here is tarjeta de crédito o de debito. Since we have two cards here, this stands for the feminine plural form, then we choose the direct object pronoun las to replace this. Lastly:
Julián: ¿Cómo vas a pagar? ¿Tienes la tarjeta de crédito o de debito?
Rafael: Sí, tengo las dos.
