Which part exactly?

Re-loading the page on navigating back would be done using cacheability headers. This is the most shaky part, and I'm not sure if it is possible today. If this does indeed not work, then this would be one of the "things that Javascript has solved that the non-JS web is still stuck with" I meantioned in my other post, i.e. one of the reasons that JS is more popular than non-JS pages today.

Identifying the order using an ID in the URL is standard practice everywhere.

When the order page gets requested, the server would take that ID, look the order up in the database and see that it is already submitted, responding with a page that has its submit button disabled.