I use the default shift key function of the mini-pac (shift-B1/2 = P1/2 credits)
But I have kids over that I would end up hidding that feature and just want to set it to free play. I'll end up mapping 2 unused keys to double up as P1/P2 credits in MAME and then also reprogram the shift key in mini-PAC to match. That way, if I was to disable that feature, I can just use a different MAME configuration file.