Should one recite Ayat al-Kursi over every new item purchased, even if it is a shoe, or is it sufficient to say: “Oh Allah, I entrust this shoe to Your safekeeping”?

It is not from the Sunnah to recite Ayat al-Kursi or any other part of the Qur'an over a new item that a person purchases. Instead, a Muslim should thank Allah for the blessing and use it in obedience to Him. The Sunnah contains specific supplications for thanking Allah for blessings, such as the supplication upon eating food or wearing new clothes, where the Prophet, peace and blessings be upon him, would say upon wearing new clothes: "O Allah, all praise is due to You. You have clothed me with it. I ask You for its good and the good for which it was made, and I seek refuge in You from its evil and the evil for which it was made." Furthermore, dedicating the recitation of specific verses or surahs for particular needs without evidence is considered an innovation.