If you have a label printer with a DB9 connector designed to be connected directly to a PC, then it is speaking "rs232" signalling on it's serial port.
RS232 is "inverted" compared to "TTL Serial" - the "1" state (including idle) is supposed to become -3 to -15V, and the "0" state becomes +3 to +15V. Because of the way that rs232 receivers are implemented, it USUALLY works if you just send "inverted TTL" (0V for "1", 5V for "0"), but that's not technically correct. (Note that Due will be providing 0V/3.3V, which is even more questionable.)
I purchased a sparkfun RS-232 level converter and that didn't work.
A level converted like that is the correct solution, and IMNSHO you should be debugging why you weren't able to get it to work, rather than chasing obscure uart options that aren't "right" anyway.
That particular converter seems to assume a bidirectional serial connection; it sucks the higher voltage for transmitter from the receive data signal. This is "clever", but it won't work if you don't connect both signals, or if the devices is truly "receive only" or if both sides are trying to do the same thing, or ... other circumstances.