UPS and FedEx shares fell Thursday after a Bloomberg News report said that the online retailer is experimenting with a new delivery program called "Seller Flex" and that it plans a broader rollout in 2018.
Bloomberg said with the project Amazon would directly take over functions typically handled by UPS and FedEx.
Amazon distribution threats can have real power, Cramer said on "Squawk on the Street." He explained that the stock of drug store retailer Walgreens has struggled after news that Amazon could be looking to break into the multibillion-dollar pharmacy market.
When asked whether UPS and FedEx investors should be concerned about Amazon entering the market, Cramer said "yes."
"We'll wait for a second day. See if anyone joins or downgrades these and then you got your shot," the host of CNBC's "Mad Money" said, referring to UPS and FedEx.
"Let's watch the transports," Cramer added. "Because classic Dow theory says as long as they go up we're fine. Watch JB Hunt, XPO Logistics — not in the transports average but [has] been extraordinary."