Unfortunately I'm not surprised by the extra charges for weekends. From my experience of corporate FBOs weekend call out/supplement charges are common practice even if there are plenty of other aircraft around. The most I've heard for 150 litres of Jet was around £1300 with call out charges and Airport fees. However I have always been made aware of what charges I will be faced with before booking in. With regards to cash only I wouldn't deal with any aviation organisation that doesn't take card payments. Maybe the tax man will be interested in their operation!