Should balance the transaction fees and cost! Otherwise people is unwilling to use your machine if charge high fee or you make a loss if you charge less.
fee's or discounts are irrelevant with bitcoin.
for instance i can find the most expensive exchange and tell people that is the valuation for them to buy bitcoins. and they would see that as fair value as that is the exchanges price (not showing fee on top).. yet actually exchange on a cheaper one. where i keep the difference in the middle.
its a known standard that bitcoin prices are VARIABLE between different exchanges AND VOLATILE between each hour so a 5% difference is acceptable. infact as i said in last paragraph. the customer doesnt even really need to be told what the fee was, especialy when having the convenience of cold hard bank notes<->bitcoin.