First part:
installing DB_cart class for existing website with data loaded dymicly from mysql
See text file for this DB_cart class
This universal shopping cart script is powered by MySQL and works with external customer and product data.
Or you can use a simular script.
Second part:
implementation in existing website with use of the existing users and products loaded dynamicly from mysql.
(we can help you by giving the mysql table data for that.)
must work same way as e-junkie which is with lightbox.
needs to work in all section of the website, and possibility to use direct checkout with several payment processors (already in the system)
I can provide more details if you pm me.