Enhancements to PHP eCommerce Site

We have a little urgent requirement from one of our clients. I am not having bandwidth in my team to get this done, so I need help from an online contractor. I need a couple of enhancements in an existing PHP ecommerce website: 1- Need to integrate PayFlowPro, so payment process should happen on our own site. 2- Enhancement: Example: Suppose we have two products, Product-1 100 ML price 125.00 USD, Product-2 125 ML price 175.00 USD. On admin side adminstrator associates Product-1 with Product-2 and enters additional cost as 25.00 USD. If a customer buys Product-1, system should show him a popup showing "Upsize your purchase to Product-2 125 ML just by paying 25.00 $ extra and save 25 $. (should show the product image)". If user chooses to upsize, system should add 25.00 $ to the originally selected items price and should also replace the Product-1 100 ML with Product-2 125 ML. So this way, customer will buy a 175.00 $ product in 150 $. Need to provide an upsize feature on checkout page with below details: Implementation Work Flow Admin Side Change: Need to add a new page on admin side with below functionality: 1 - Admins will select a product from a dropdown 2 - Admin will select another product "Upsize product" from a dropdown or live search box 3 - Admin will write additional cost "Upsize Cost" in a text box 4 - Save the page Customer-side checkout Flow: 1 - On checkout page, we will show a pop up before final checkout. This popup will show the associated upsize items with the product that user has selected to buy. 2 - If user chooses to upsize his purchase, on final checkout page originally selected item will be replaced with the selected upsize product and given amount will be added to the invoice. 3 - Business rule is the popup will only show if user has selected only one product to buy. 4 - Upsized flag and uszied price should also save to Order table for reporting and future references. Note: Developer will be responsible for the successfull deployment of the changes and 2 weeks bugs warranty/support. I will not provide any credit card info for local testing, you have to manage that by yourself.