What is the upload_path for addon domains that are not in public_html

I recently moved an e-commerce installation from a shared host to an add-on domain on a VPS. My original shared host install was: https://www.domain.com/wordpress/

The new VPS host is https://www.vpsdomain.com where I created an add-on domain: domain.com (same name as above). I did all the DNS/nameserver stuff.

The add-on is NOT in the vpsdomain.com/public_html directory. I set it up one level higher so that my url is simply https://www.domain.com

I transferred the wordpress installation and SQL database from the shared host to the VPS. I put wordpress in https://www.domain.com/wordpress/ and made the necessary change in the index.php file

My problem seems to be with the upload_path option in the database.
I have it set as:

/home/username/domain.com/wordpress/wp-content/uploads

I think that path must be wrong because when I do a test paypal purchase on the site I get the following error when paypal returns the customer to the site:

Warning: require(./wordpress/wp-blog-header.php) [function.require]: failed to open stream: No such file or directory in /home/username/domain.com/wordpress/index.php on line 17

Everything else on the site seems to work normally. Because domain.com sits above public_html I’m confused about what the path should be. Sorry for the long-winded explanation. Any thoughts are welcome.