OpenCart PHP Store software modify file uploader/ fix erorr

**Alright I installed this PHP store software called OpenCart yesterday and I like to keep using it. The only thing is I sell downloads products like ebooks and mp3 music files now and I like to use this store. When I try to upload a zip file to store it in the download folder I get the following errors:** Notice: Undefined index: download_description in /home/gtstars/public_html/girlsstock/admin/controller/catalog/[login to view URL] on line 420Warning: Invalid argument supplied for foreach() in /home/gtstars/public_html/girlsstock/admin/controller/catalog/[login to view URL] on line 420Notice: Undefined index: download in /home/gtstars/public_html/girlsstock/admin/controller/catalog/[login to view URL] on line 426Notice: Undefined index: download in /home/gtstars/public_html/girlsstock/admin/controller/catalog/[login to view URL] on line 25Notice: Undefined index: remaining in /home/gtstars/public_html/girlsstock/admin/model/catalog/[login to view URL] on line 4Notice: Undefined index: download_description in /home/gtstars/public_html/girlsstock/admin/model/catalog/[login to view URL] on line 12Warning: Invalid argument supplied for foreach() in /home/gtstars/public_html/girlsstock/admin/model/catalog/[login to view URL] on line 12Warning: Cannot modify header information - headers already sent by (output started at /home/gtstars/public_html/girlsstock/admin/[login to view URL]) in /home/gtstars/public_html/girlsstock/system/engine/[login to view URL] on line 28Warning: Cannot modify header information - headers already sent by (output started at /home/gtstars/public_html/girlsstock/admin/[login to view URL]) in /home/gtstars/public_html/girlsstock/system/engine/[login to view URL] on line 29 **I want to try and redesign this file up loader that stores these files in the download folder, make it more flexible. I want to sell downloads like MP3 and ebooks stored in a zip folder. What you will get is the original opencart php store software to work with. I downloaded v1.5.2.1 at: [login to view URL]**