I think you'd want to use E-junkie for the initial purchase and sample download, then you could use any more direct method to provide the buyer with an unlimited link to their customized download.
You could just park the custom download file on your own Web site's server (maybe putting such files in a special subfolder or sub-subfolder such as www.yoursite.com/clients/lastname) and give buyers the direct URL to their file, or (especially for large files) use a generic file storage/sharing service such as MegaUpload, RapidShare, MediaFire, SendSpace, etc.