Add E-Commerce functionality to Premium Wordpress Theme

Hi, I purchased a premium wordpress theme from themeforest. Every page on my current site is static except for one page (and sub pages) which will be a small online store (10-20 Items). It must have a few categories and then Visa/Mastercard or paypal payment methods and a shipping calculator. In the top header which at the moment only has an email address and phone number to have a shopping cart at all times. This will be the only part of the online store which is on every page. Attached is an example layout which I like the shopping cart design in the header except not the dynamic shopping display on all pages. The actual theme used is similar to the screenshot althoughw will be discussed with the winning freelancer.