"hidden extras" means additional costs or things that are not mentioned upfront or easily seen, but you have to pay for or deal with later. Full definition
Unlike some wallets that make a show of hiding extra pockets, this one has two simple slots — one for you cash and one for your ID and credit cards. (businessinsider.com)
Check their itineraries carefully and read the small print for hidden extras. (travelfish.org)
They will hide the extra pounds and look much more interesting than skinny options. (howomen.com)