Hello,
Could you further clarify on what it exactly is that you need by updating the project description?
I may be able to do this project if I can figure out what it is exactly about.
First off, what does this "extension" refer to. My guess is that you have an existing site and want a parameter in the url that says something like "&product-batch-size=5" and then on your site a button that adds/substract 5 products at a time / only submits if the product count field is cleanly dividable by 5?
Also the project is listed as both C# and PHP, is there an existing site you need altered or do you need a new one build. If you already have one it would be quite unusual to need both C# and PHP.
My apalogies if I come accross as rude, I'm just trying to determine what you exactly want doing :).