For some time, the US Postal Service has allowed people to generate barcoded postage. You can do that on the expensive forms of mail such as priority mail and express mail, but if you want to do it on ordinary mail, like 1st class mail or parcel post, you need an account with a postage meter style provider, and these accounts typically include a monthly charge of $10/month or more. For an office, that’s no big deal, and cheaper than the postage meters that most offices used to buy — and the pricing model is based on them to some extent, even though now there is no hardware needed. But for an ordinary household, $120/year is far more than they are going to spend on postage.
There is one major exception I know of — if you buy something via PayPal, they allow you to print a regular postage shipping label with electronic postage. This is nice and convenient, but no good for sending ordinary letters and other small items.
I think the USPS is shooting itself in the foot by not letting people just buy postage online with no monthly fee. The old stamp system is OK for regular letters, and indeed they finally changed things so that old first class stamps still work after price raises, but for anything else you have to keep lots of stamps in supply and you often waste postage, or make a trip to a mailing office. This discourages people from using the post office, and will only hasten its demise. Make it trivial to mail things and people will mail more.
It could be a web printed mailing label as you can use for priority mail, but most software vendors would quickly support such a system. If people wanted, they could even buy “stamps” which were collections of electronic postage in various denominations that could be used by programs so there is no need to handle transactions. Address label printers would all quickly also do postage.
Of course the official suppliers like Endica and stamps.com would fight this completely. They love being official suppliers and charging large fees. They have more lobbying power than ordinary mailers. So the post office is going to quietly slip away into that good night, instead of taking advantage of the fact that it’s the one delivery company that comes to my door every day (for both pick up and delivery) and all the effiencies that provides.