When we send a letter or a birthday card to a UK address today, we pay postage based on the urgency of delivery. 1st class stamps (currently beginning at 67p) should deliver the following working day. If the item is not critical, such as a birthday that is several days away, we may pay for the slower, 2nd class stamps (currently starting at 58p). This should result in delivery on the second or third working day following posting.
Before the Post Office introduced the two-tier system for inland letter mail on September 16, 1968, the sender’s choice of service was determined by the contents of the envelope or wrapper. The contents of the package determined the convenient Discounted stamps (and different tariffs). Perhaps the sender erroneously agreed that goods sent at the discounted rates would be delayed until the fully paid letters were delivered.