Agreed – it would be nice.
I just cannot imagine asking for a 20pound voucher unless I wanted just that. Otherwise I'd ask for 20 pounds worth of vouchers.
And I cannot imagine that there would be a barrier to making a transaction because the wording of my request did not compute with the shop assistant in the first place. Had I anticipated that she would struggle in selling me vouchers to the value of £20 unless I specifically asked for them in that manner, then maybe I would have proceeded differently.
And still, after all these responses, those sitting on high are still missing the fact that the original post was only ever meant to be said in jest.
But that is a fact that doesn't really matter to those righteous ones amongst us.