Do the business reply envelopes ever expire or become obselete?
No. You only pay for the postage as it is used, so if there is a postal increase in the future, the current rate will be charged when the envelope is used. They never go out of date. However, the self adhesive gum used does eventually dry up, so it is best to only order enough envelopes to last you about twelve months. Your licence will also need renewing every year, which Royal Mail will remind you about.
