My guess is the £3.50 is because the seller isn't aware of the lower cost on postage offered by RM at the moment. But then maybe they will give ALL proceeds to the charity in the listing. Likewise, the £3 postage on another listing maybe to help cover fees/packaging.
I stated earlier that postage should be at cost but how do we cover postage/packaging when we don't have a bottomless pit of cash to spare?