What is the ruling on selling fast food to a customer who does not ask about the price until after completing, receiving, or eating their order?

"Bay' al-mu'atah (exchange by mutual consent) is permissible according to scholars. This is supported by the statements of فقهاء (jurists) like Imam al-Quduri, Ibn Taymiyyah, Ibn al-Qayyim, and Ibn Rajab. It is also indicated by people's dealings and customs. Therefore, if someone buys something from a grocer or similar vendor and does not pay immediately, there is no harm in that, as long as they took it with the intention of buying. When they do pay, the price should be based on the price of the day the item was taken, not the day of payment. Istisna' (manufacturing contract) is also permissible if it conforms to the agreed-upon description."