FBA Gap Auditor
Amazon's own reports go in; a worklist of gaps comes out, each with the time left to file.
A full worklist of gaps with a countdown on each, for the first 200 reimbursement rows.
FBA Gap Auditor is free to use today, and there is nothing to buy. It is planned to be part of the $19/month Avyth subscription, which covers every tool with a recurring job — Invoice Splitter, SlipMatch, FBA Gap Auditor and Article 50 Check — not a separate charge for this one tool — it would add: Claim templates, calendar export and unlimited rows.
It isn't built, so there is no checkout on this site and no way to be charged. When it exists it will appear on the updates page, with the date.
Email support@avyth.com and the person who built FBA Gap Auditor reads it. If it fails on your file, say what the file looks like — that is usually enough to fix it.