the bill in the mail
doesn’t say a word about Hal’s devolution from occasional tourist to subterranean compulsive, substance-wise, with his Pump Room visits and Visine, even though Schacht deep down believes that the substance-compulsion’s strange apparent contribution to Hal’s erumpent explosion up the rankings has got to be a temporary thing, that there’s like a psychic credit-card bill for Hal in the mail, somewhere, coming
Context
The scene's quiet prophecy: Schacht watches Hal cross the same line Schacht himself never approached — "occasional tourist to subterranean compulsive" — and reads the ranking-surge that the habit seems to fuel as borrowed money, a psychic credit-card bill already in the mail. The compulsion's apparent gift and its certain debt named in one breath, years before the withdrawal that will collect.