Kucoin sometimes gives a limited discount offer of 99% for about 10 days for trading new coins which inludes the recently listed stablecoins like Paxos standard. Read here for more info https://news.kucoin.com/en/kucoin-will-add-paxos-standard-token-pax-market/
After the ten days expire you will be required to pay normal trading fees.

On Huobi's end, its HUSD stablecoin is a permanent integrated solution that eliminates the costs of switching between stable coins or the inconvenience of choosing between them. However, trading fees between HUSD pairs are standard exchange fee charges unless a special offer is announced.

I hope you get the difference :- )