Hill fixed Britain's mail by charging the sender

Before 1840, British postage was charged to the person receiving a letter, not the person sending it, with the price set by distance traveled and number of sheets rather than a flat rate. This created an odd incentive structure: a recipient who couldn't or wouldn't pay could simply refuse delivery at the door, leaving the Post Office having carried the letter across the country for nothing, and correspondents exploited the system by encoding messages in the address itself so the recipient could read the information and then refuse the letter to avoid paying at all.

Complex distance-based pricing also meant nobody could predict in advance what a letter would cost either party, and a poor family could be handed a delivery bill for an unsolicited letter they had no way to anticipate or budget for. Reforming the price alone wouldn't have fixed the underlying mismatch: whoever was billed still hadn't been the one to decide whether the transaction should happen in the first place.

Charged the sender upfront with a flat rate, proven by an adhesive stamp bought in advance, so payment happens at the moment of sending and refusal becomes impossible.

Charging the sender upfront removes the recipient's refusal option, so the Post Office is paid regardless of delivery.

The sender, who initiated the letter, has no incentive to refuse, eliminating the address-encoding dodge.

A flat prepaid rate made sending predictable and settled, boosting mail volume as transactions became risk-free.

When the billed party isn't the one who chose the transaction, move the charge to the chooser and collect upfront.

Paid internal mail volume rose roughly 120% in the few months around the penny rate's January 1840 launch, and the total number of letters sent in Britain more than doubled within that first year and doubled again within a decade — because the party being charged was now the same party who had decided to send the letter, refusal at the door was no longer a way to dodge payment, and the Post Office collected its money upfront instead of chasing it after delivery.