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[Moved the discussion to InDesign Forum]
There is no direct export capability to PDF/A in InDesign. This is not a common workflow. Export as a PDF that creates the compression settings for images that you need.
You'll have to use Acrobat Pro to convert this PDF to PDF/A. I'd suggest using Acrobat's Preflight tool. There are profiles to do this in the Preflight tool. Then you can create a Preflight droplet from the Preflight panel menu. You can drag PDF files onto the droplet to automate the process.
Note that PDF/A certification can fail on account of XMP metadata that ID exports into PDFs, as described in the Acrobat forum. In particular, the category xmpMM:History can cause problems, as described by Dave Merchant, who provides a link to his scripts that add a menu items to Acrobat 9 and X for stripping xmpMM:History.