Modify function output in a plugin

You haven’t created your filter yet.
You can’t just filter a function.

You’ll need to call the function apply_filters(), and then you can use it in your functions.php file.

So above return $content; add something like this:

$content = apply_filters( 'my_content_filter_name', $new_content, $content );

Then, use it like this:

function modify_store_output( $content ) {
    $content = $contact . $address . $filter . $map . $content;
    return $content;
}
add_filter ('my_content_filter_name', 'modify_store_output', 15, 1);

Notice how $content is passing through the filter to the input of your modify_store_output function. Also, the 1 at the end of the filter is how many of these variables (arguments) you are passing through. In this case it is only one, but if you wanted to pass more arguments to your filter function you would increase the number to however many you are using in your add_filter() callback function.