Your displayed prices on STF are applied for STF members. Charges for non STF-members are set up on your BookVisit account and are on STF’s booking page added on top of the displayed prices. The prices you send to BookVisit must therefor exclude the charge for non STF-members.
The surcharge for non STF-members is 50:- / adult, so you have to adjust the prices for your room types connected to BookVisit like this:
Single room – 50:- lower
Double room – 100:- lower
Tripe room – 150:- lower
And so on.
You need to have unique room types for the connection to BookVisit in order to achieve this. Copies of your already existing room types can easily be set up, by creating duplicate room types. See this support article for instructions on how to create a duplicate room type.