A "Make Up Rate Stamp", sometimes also called "additional postage", is a stamp issued by the United States Postal Service to make up a slight difference in postage rates, either because the postal rate has changed, or the customer needs to pay some type of additional fee. The usage of Make Up Rate Stamps has decreased greatly since the introduction of the forever stamp. The Make Up Rate Stamps were definitives, and were generally plain in design, and there were not generally seen as collector's items, even among the often obsessive hobby of philately.
The United States Postal Service still sells 1, 2 and 3 cent stamps, including in sheets of 20, but also in bulk, with the US Postal service having introduced, in 2016, a new 1 cent stamp featuring apples, available in coils of 3000 or 10000. Apparently, there are businesses that still have hordes of postage stamps from before the introduction of the Forever Stamp, or have other unusual postal needs, that can only be addressed by pruchasing 1000s of 1 cent stamps.
Although the reason for having make up rate stamps has mostly been lost, I do remember the fun or ordering various different odd denomination of stamps and then covering envelopes with 5 and 1 cent stamps to equal first class postage.