I have created a class for PayPal that works great for adding single items however I am now trying to get this working as a cart so i can add multiple products and customise
I have had this same issue in the past. What I had to do was create my own cart independent of Paypal. Then, when the customer was ready to check out, I forward them to Paypal. Instead of sending multiple items and prices, I combine everything together for Paypal.
For example if the customer orders 3 different items @ $5 each, my site would show 3 items and 3 prices. However, upen being forwarded to Paypal, the customer would see one item (ie "My Company Order") and one price (ie $15). But when returned to my site, the customer would once again see that there were 3 items ordered.
If you are looking for a good cart system, there are literally hundreds available that support Paypal and this method.