Whether the sale or service is made to an out-of-State customer is a question of fact. In order for a customer to be considered an out-of-State customer, some specific relationship between him and the seller has to exist to indicate his out-of-State character. Sales made to the casual cash-and-carry customer of a retail or service establishment, who, for all practical purposes, is indistinguishable from the mass of customers who visit the establishment, are sales made within the State even though the seller knows or has reason to believe, because of his proximity to the State line or because he is frequented by tourists, that some of the customers who visit his establishment reside outside the State. If the customer is of that type, sales made to him are sales made within the State even if the seller knows in the particular instance that the customer resides outside the State. On the other hand, a sale is made to an out-of-State customer and, therefore, is not a sale made “within the State” in which the establishment is located, if delivery of the goods is made outside the State. It should be noted that sales of goods or services that are conditioned upon acceptance or rejection by an out-of-State source are interstates sales and not sales made within the State for purposes of section 13(a)(2). For example, a contract entered into in the State where the customer resides for the delivery of a magazine to the customer's residence, is an interstate sale if the contract must be approved by the out-of-State home office of the company publishing the magazine before it becomes effective.