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 <em>tarjeta de crédito o de debito. </em>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:
<em><u>Julián:</u></em><em> ¿Cómo vas a pagar? ¿Tienes la tarjeta de crédito o de debito? </em>
I would add me gusta and no me gusta before the ones you like and don't like. Gusta means i like and no me gusta means i don't like so you will be golden!!