I need a Bigcommerce expert to code a product variant

I have a new BigCommerce (Cornerstone) site and I need some expert programming to set up a custom product variant based on user input. I'm told it can be done but not as part of my standard BigCommerce support. My e-commerce site needs to accept a user input quantity . The quantity on offer to customer will be between 0.25 and 20 in .25 increments. All possible increments are acceptable as long as they are checked against stock level. Price calculated based on this quantity.
I know that I can get around this by creating every possible option and letting user choose bit I would prefer a smoother/sleeker page. I do also require some theme tweaks if you have those skills as well