BigCommerce - Build a Sample Picker

Hello, We currently run on BigCommerce and are looking for a developer to build out a sampling program. This is the structure of the program; 1) There will be 10 products that are in a single category that are all samples (these are available for purchase at regular price) 2) If a customer has $35.00 in their cart, they are allowed to choose a single sample from that category page for free. If a customer has $50.00 in their cart, they are allowed to choose two samples from that category page for free. If a customer has $70.00 in their cart, they are allowed to choose three samples from that category page for free. 3) When a customer adds products to their cart and reaches the threshold, there should be a popup stating, 'pick your free sample' -- from which they can choose to add the samples directly from the pop-up (as opposed to redirecting to the category page) 4) On the checkout page, they will also have the option to choose their sample Of course after they choose their free sample, all the other samples will be grayed out (i.e the 'add-to-cart' button and at the bottom will be a button saying 'checkout' They will also have the option to remove the sample they selected and pick a different one. Must be familiar with stencil API, BigCommerce, Designing and UI/UX.