A small EPUB repair tool: free diagnosis, $7 for a supported repaired download

I'm building EpubMend for people whose EPUB files fail validation because of a small set of structural errors. The question I'm testing is whether avoiding editor installation and manual archive fixes is worth a one-time payment.

The check and validation report are free. A repaired download is $7 plus applicable tax, and is offered only after the repair passes EPUBCheck and content-preservation checks.

The most important design decision was what the tool must refuse. It can fix mimetype ZIP packaging, rebuild a missing container pointer when one package is unambiguous, and correct certain EPUB 3 image media-type declarations. It does not reconstruct a missing chapter, rewrite the manuscript, or promise bookstore acceptance.

For a concrete example, a synthetic EPUB tested today went from two packaging errors to zero. The other five archive entries, including the chapter and image, were byte-for-byte unchanged. The same example was checked through the live UI. No customer file or paid transaction was used for that demonstration.

The service is here: https://epubmend-production.up.railway.app/

I'd particularly like feedback from anyone who has built publishing tools or prepared EPUBs: are these errors frequent enough to justify a dedicated service, and what evidence would you need before trusting it with an unpublished book? Please don't share private manuscripts in a public reply.

Disclosure: AI assisted with development and drafting this post. The example and repair limits above were checked against the running service and its implementation.