The store page is showing an archive of, whatever you have WC to show own the archive. The page is really just a placeholder that when that page is loaded WC intercepts the main query and alters it so that it is no longer for the page but for an archive.
You need to provide a post ID value when getting the fields on this page. First you need to figure out how to detect that this is the page being shown. It may still be in the main queried object.
$queried_object = get_queried_object(); echo '<pre>'; print_r($queried_object); echo '</pre>';
If this works then you can get the post ID from there.
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