In 1837, British postal rates were high, complex and anomalous. To simplify matters, Sir Rowland Hill proposed an adhesive stamp to indicate pre-payment of postage. At the time it was normal for the recipient to pay postage on delivery, charged by the sheet and on distance travelled. By contrast, the Penny Black allowed letters of up to 1⁄2 ounce (14 grams) to be delivered at a flat rate of one penny, regardless of distance.
The receiver had to pay and not the sender! So a letter of two pages travelling one hundred miles would cost 18 pence or one shilling and six pence. From 1840 the same letter if it weighed under half an ounce cost the sender just one penny.
The introduction of uniform penny postage resulted in increased trade and prosperity, with more people sending letters, postcards and Christmas cards than ever before.
According to bl.uk & en.wikipedia