just got it wrong.

Most items of ordinary post were paid for by teh recipiant.You could pre-pay items.saee ths quote from The British Postmarks a Short History and Guide by Alcock and Holland;

A large percentage of General Post letters were paid for on delivery by the
addressee, but it was possible also for the charge to be pre-paid in cash
by the sender. The fee pre-paid was at first written on the front of the
letters, but quite early in the 18th century the PD type began to be used
in black on paid letters to the Continent. By 1740 it was also impressed on
paid inland letters. In 1766 a new design replaced the first type. in
1788 dated marks followed first used on evening duty on paid letters sent
from the London Chief Office. Special marks for prepaid letters were used
by English Provincial offices at least as early as 1795."

So I suppose a rich /considerate person with access to the post Offices would have used this system so as not to impose upon the sender.