You tried it with and without a space after the third keystroke?
Find the postal code for the nearest municipal building (firehall, police station, city hall, whatever). Use that postal code.
Did that help?
Note the correct code in the text box on the invoice when you pay.
Don't panic.
One of the job requirements for letter carriers is literacy. At sorting the incorrect code will be noted and corrected, although it may slow delivery down a bit.