I need help to fix my errors in my ERD diagram. ERD is about system delivery. I got issues that i have to consider all the FKs in Items table and not null. like

What if a customer only buys gold , then how to handle the rest of PKs' values.

i need to have the superclass and subclass concept for those items. i have to

create a superclass that is associated with items and subclasses with unique attributes.

i should have major relationships that will be associated with the superclass.

my other issue is delivery table, it needs a relationship with CUSTOMER.

Supplier shipping Item to Customer.

the diagram is below, I couldnt download the image.

https://www.draw.io/?client=1&title=Copy%20of%20Untitled%20Diagram.pdf#LSSRm