score:4
I managed to contact them through their Facebook page.
They say:
the provided service and checked baggage is the same, no matter which
the fare is, as we mentioned earlier, the only difference between fares
is the percentage of discount in the fare.
So it seems to be "pick your own discount" type of thing. It really is puzzling to me as to why would anyone pick anything other than the lowest possible price, but apparently offering this kind of choice makes sense to somebody out there.
Update 2015-10-01: Have had 2 flights with BoA now and the cheapest fare worked out fine, luggage (up to 23kg) was included, no other difference in service either. I dont know about refunds though.