Unless you're going to dispense with the handling charge and instead add it to each of the weight / price range figures, then don't know if this works for you but I tried an experiment using table rate and setting a negative weight of -2 for the gift voucher.
I then set the table weight range using a minus value equivalent to the handling charge which then removed it. eg 1:-2.00. Or, you could set the minus value to adjust the handling charge to the amount you actually want to charge for the postage. Stays the same regardless of quantity although I imagine this isn't an issue with this particular product. If other products are bought (assuming a weight value is set) then the normal handling charge is applied.
Tried various other values but it wouldn't work so don't know what determines these figures or if these values would work for you. It's a bit of a fudge and might look a bit odd if you show the weights in the cart but seems on the face of it to work.
Might be a way to use attributes otherwise...maybe some of the clever bunch around here might have another suggestion.