Hi Sir,
I Have read your project description. I understood your requirment. i have 6 year exp. in opencart. All my completed projects on freelancer.com are on Opencart. You can see my profile to know clearly about this: https://www.freelancer.com/u/sprextech .i can add it option inside the admin product.i can do that job perfectly.
I want to discuss with u more project detail , so please message me on chat .
Hope to work with you.
Thanks
Sprexe Technologies