Shopify Sahara Theme Customization - Programmer

I’m building an e-commerce site on Shopify and have already settled on the Sahara theme, but I need a programmer to push it the extra mile. My priority is a “quick add” or promo popup that automatically detects and displays correctly on every screen size—desktop, tablet, and mobile—without slowing the store down.

Currently, quick ad automatically ads a large we can not have this. It needs to have a pop up box to select the size to add to car. This is needing to be done By Thursday

A short outline of your approach and a realistic timeline will help me choose quickly. Looking forward to seeing what you can do.