As the U-S Postal Service considers cutting its delivery week from six days to five, Iowa Senator Tom Harkin is offering an amendment today that strongly opposes any such cut in service. He says eliminating the sixth day would lead to delays in mail delivery as well as higher costs to pay overtime postal workers. Harkin says his amendment would call on postal service officials to insure there is not an interruption in the essential service of delivering the mail. Ending Saturday delivery would require approval from Congress because the Postal Service is still considered a government agency.