Affirm CEO Max Levchin said Friday that while the buy now, pay later firm isn't seeing credit stress among federally employed borrowers due to the government shutdown, there are signs of a change in shopping habits."We are seeing a very subtle loss of interest in shopping just for that group, and a couple of basis points," Levchin told CNBC's "Squawk on the Street."At least 670,000 federal employees have been furloughed in the shutdown, and about 730,000 are working without pay, the Bipartisan Policy Center ...