Miami—The U.S. Postal Service's efforts to trim costs by closing processing centers, which would eliminate first-class next-day delivery, could cost large companies as much as $100 million a year each by slowing payments, according to a new study from cash-flow consultant REL Consulting.

Last month, the Postal Service announced that its plan to close about 300 of its nearly 500 mail processing centers would eliminate the possibility of next-day delivery of first-class mail.

According to REL, the elimination of such delivery is likely to add at least two to four days to the collections cycle for many companies. REL recommended that companies bill more frequently, prioritize collections and encourage electronic payments.