Good find as there is no info on this anywhere else.
They did not update MP flagging to reflect this better - and since there are no other way of purchasing than inworld, inflation shouldn't even apply to SL. Why even have it in listing guide?
On side note I would avoid shops that inflate the price, I never purchase from those that charge 10% simply because I don't think I should be covering their fees, that's like asking customers to pay your taxes. Just raise your prices if you want to cover higher fees, like in real world. MP is convenient quick way of shopping and I wouldn't pay extra as a customer to a store that requires me to go to inworld to pay less. It seems unfair to people who prefer MP shopping.