It's not as complicated as it seems from a technical perspective. Stamps have been UV-tagged since the 1960s as an anti-counterfeiting/automated detection measure. In the analog era, you shine UV light on a letter and if it it glows enough, it's got a stamp.

Modern processing machines (AFCS) capture both a visible and UV image of the mailpiece, so it's fundamentally changing the image processing software to have it look for the barcode on the UV side if it can't find one on visible. The downstream DBCS sorters already read secondary barcodes that are applied by the AFCS which happen to be in UV ink.

Further, USPS benefits from the same idea to hide a barcode for marketing (junk) mail. When combined with a handwriting-like typeface, it makes letters look personal and gets people to open them more. Same psychology where mailers apply a bulk-rate stamp instead of using a permit imprint.

It's a very Steve Jobs/Apple case where it sounds like a minor detail that requires crazy advances, but under the covers, it's very technically feasible, and importantly, can exploit economies of scale.