Sales to customers located in the same State as the establishment
are sales made ``within the State'' even though such sales may
constitute engagement in interstate commerce as where the sale: (a) Is
made pursuant to prior orders from customers for goods to be obtained
from outside the State; (b) contemplates the purchase of goods from
outside the State to fill a customer's order; or (c) is made to a
customer for use in interstate commerce or in production of goods for
such commerce.
