"Flex Mix" is just a term used by PreSonus to point out that there are not "only" aux mixes available on your mixer. By default the 16 mixes are aux mixes/monitor mixes, set to "prefade 1". When used as subgroups the flex mixes are simply "sub mixes", which means that you can add all drums channels to a stereo subgroup and control the whole kit with a stereo fader. "Matrix mix" means that the mix contains all inputs and outputs, including the Main mix. Sometimes complex control mixes or "feeds" are needed.
The UC Surface Manual explains the "Aux Send Position" (section 2.4.1):
"Pre 1: Sends each channel to the aux bus after the polarity invert, high-pass filter, and gate.
Pre 2: Sends each channel to the aux bus after all Fat Channel processing (polarity invert, high-pass filter, gate, compressor, EQ, and limiter) but before the fader.
Post: Sends each channel to the aux bus after all Fat Channel processing (polarity invert, high-pass filter, gate, compressor, EQ, and limiter) and after the fader."
What you need to do is this (stereo mix as example):
1) Select Aux 1 and link it with Aux 2 to a stereo mix
2) Go to settings and change the mode of the mix to postfade (linked Aux 2 is automatically included)
3) Raise the faders of the three or four signals you want to SEND to Skype to Unity
4) Raise the master fader of aux 1+2 to Unity
Now the "Skype mix" mirrors exactly the movements of the faders in Main mix L/R.
Just as an explanation: You can not set input channels to postfade, only aux mixes.