Save them as PDFs and store them in your cloud storage or choice or a syncing tool like Syncthing. A basic folder structure can help keep things organised.
No need for anything complicated, for essential documents best to keep its impel and limit the scope of failure.
The Amazon checkout has become a real maze for non Prime subscribers, about time something was done.
I cancelled Prime a while ago, and the few times I’ve used Amazon since I got multiple checkout stages with pre-selected Prime signups and trials.