I’ve learned this from an US FDC dealer: “Uncacheted FDCs that have an added cachet is usually called an “add-on” cachet. These are done after the stamp has been issued – usually several years later. This cachet can be either hand painted – hand colored, computer generated, rubber stamp ink or sticker added. However created, the cachet “enhances” the cover and usually makes it quite attractive. These add-on cachets are usually done in small quantities and thus are really quite scarce! The value of an add on cachet FDC is simply determined by the market place – what one will buy. Many artists are now painting cachets on older issues and have quite a following. Auction prices can be quite high!” (Source: US FDC seller dwa7).
Ben Kraft has made handpainted add-ons on US FDCs spanning from 1940 to at least 1990. Some of them are possible to obtain for affordable prices, others are quite rare like this SA cover. The SA cover has a picture of Mickey Mouse as Santa Claus with an SA Christmas kettle.