It depends on the type of metafield you add. A zip is stored as a string, so I would use a string or list of strings (with one zip per item in the list).
Once you have that set up, you can add the metafield to flow by going to “shop / metafield (requires arguments)”. There, you’ll be able to choose that metafield. If you use a list, you’ll probably want to add that first in a condition (because you need to loop over the list). Then add the whichever zip as the second part of the condition.
FYI, there’s isn’t yet a UI to set shop metafields…but there are some free apps that allow it like “metafields guru”.