I’m not sure but if one used the USPS on line shipping feature any shipping label printed out would have a bar code. One would need an accurate scale for weight of course.
The USPS is big into the modern age of computer use and on line business transactions must be a cake walk, for instance I started using an app on my phone called “Informed” delivery. I get an email daily telling me what mail and packages are being delivered the next day. Photos of the items included.
I live between two address Winter Spring. Summer Fall, Informed delivery follows me.
You can't use the USPS website to print labels for Media Mail. It's not an option. I've never used Stamps.com, you MIGHT be able to do so there.
Every package is supposed to be scanned, multiple times. A Certified letter dropped in a collection box stands a VERY good chance of not having an intake scan.
I was a letter carrier for 20 years. I had a couple of collection boxes on my route. One got around 20-30 letters on a normal day. The other got hundreds. I wasn't given the time to sort through all those letters to see if they were certified. If I happened to see one I would scan it. However it was mostly a case of switch out the containers and go. When I got back to the station all my outgoing letter mail was dumped into a larger hamper. No one physically went through that hamper. That mail, for the most part, was never touched by human hands till it was delivered.
Old Lady Shooter,
If I were in your shoes, I'd make nice to your letter carrier. We have/had a surprising amount of discretion in what we could pick up. Check into some of the other online postage printing options.
I just checked Stamps.com for you. You can print Media Mail labels with their service. However there is a $17.99 per month fee. It would be up to you to decide if this works for you or not.