Millions of items sold on this platform a day.  You really think anybody is empowered to wave a wand and lower YOUR fees?  That's assuming they were inclined to do so, which they are not.  The platform fees are well documented.  You either perused them before selling or chose not to.  

Your 20% claim is likely incorrect.  Unless you've paid to promote your listing the fee will be around 13%

FB marketplace definitely has some benefits.  It also has some very real downsides (scammers, no payment processing, lack of automation / API) which make it impossible to sell at scale, but perhaps it's the right option for you.