So the gadget is producing a +-5v signal.
If you use a hardware UART you have to invert, no choice.
If you use NSS and the invert features works then
receiving
A diode and an external pull down resistor should do.
transmitting
Using nothing might work, many 232 receives will operate with a single-ended input. But it might not as well.
If it doesn't work then you need to generate a - voltage, in which case you may as well use a MAX232-style chip and be done with it.
If this is a commercial project I would recommend adding a MAX232 or similar. If you're just playing around then try it without anything on the transmitter.
Rob