ERD is about system delivery. I got issues that i have to consider all the FKs in Items table and not null.

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.