One time I needed to use Costco Photo Center to print my photos I was surprised to find out they didn't offer a Lightroom plugin to upload files to their lab. Their web site uses a propitiatory API, which they don't disclose (at least as of today it hasn't been disclosed yet). But I was up for the challenge. After a few days of battling it out with the black box of Costco web site I was able to produce this little plugin.
Download and use this plugin free of charge, all I ask in return from you is to provide some feedback.
Visit the
Export to Costco Photo Center page to download the plugin package.