Santa Barbara Books is an adult book store where Craig buys, only for cash, the child incest material which is at the core of his sexual fantasies.  The receipt was in the bag of trash, tucked into the back of the magazine/book.  These were placed by Franklin in a gift bag, used for his trash, on the top of the overfull trash can in the garbage area of his condo complex, this being seen by the private detective hired to locate him by Pillsbury-Foster.  The bag of trash was brought to her home by the detective immediately.  There, it was opened the the contents recorded.