The short version: it feels as if Amazon made it as painful as possible to do the process
First, there appears to be no standardized way of informing that the ebook is DRM-free. The examples I could find had it mentioned at the title, at either of the descriptions, and/or at the product details. Ctrl F or your browser's equivalent might help.
Second, finding and downloading your contents is rather manual. Learning that they're downloaded at the Digital Contents page (or your region's equivalent) saves a lot of time as you can then bookmark the page. And at there, you have to click on "More actions", hope the ebook actually has a DRM-free download¹, and click the option that mentions "EPUB/PDF" if available (in the case of Amazon US, "Download EPUB/PDF").
Third, Amazon JP works pretty much the same, デジタルコンテンツ (Digital Contents) page > その他のアクション ("More actions") > EPUB/PDF ダウンロード ("EPUB/PDF download"). Also, you can look for mentions of "DRMフリー", "DRMなし", "DRM free", etc., though I'd presume (since I haven't tested as much) that reliability is as similar as the US counterpart.
Fourth and continuation of the second point, retroactively looking at the ebooks in your account requires checking each "More actions" menu individually. No filters available. Also skimmed through the page inspector (F12 in Waterfox), and you can find the download options when present by searching for "EPUB/PDF", though do note there's a bit of an offset when using "Scroll into view", as it puts you where the option would be if the More actions menu was expanded, not on the height of the ebook itself.
Fifth, for Brazilians, the path for downloading is Conteúdo digital > "Mais ações" > "Baixar EPUB/PDF".
And lastly, haven't tested on other regions, though most overseas sites to my knowledge are tied, rarely there being region-specific features or contents.
¹I found two false positives while testing; ~2 USD poorer because of that but at least serves to confirm the issue.
Hopefully it can help people. =)