I'm looking for the exact name of the UI card (?) element marked with the red dashed lines here that is often used for product purchases. Does anyone know the name of it?
This is called a SlidingUpPanel - given that one has this sort of interaction behavior with the element in mind:
Searching for SlidingUpPanel as UI element delivers the best search results as well as the most ready to use components out of various frameworks.
Those are simply called "cards" in every environment that I've seen them in. You could add other descriptors in this case, such as "flat", "rounded", and "low contrast" (the text in the images you show would likely fail an accessibility audit). But they are just cards. Searching for "checkout card" or something similar can get you more examples.