I’ve written a lot about transaction charges (positive and negative) elsewhere in this forum so search around if interested.
Short answer - yes, negative transaction costs are ‘real’. It’s entirely possible to generate profit from trading activity due to the mandated calculation used in the arrival/slippage Methodology!
google it, read it and despair at what regulators think make a logical outcome and helps retail consumers to make better investment decisions!
These ongoing fees are nearly always estimates. So you’ll never truly know what’ll you’ll actually pay down to the penny. You can find ex-post disclosures - but by then it’s been charged.
Transaction costs are calculated historically - so again you’ll never know what you’re actually going to pay in the future!