PayPal/Shopping cart addition or alternative

I'm looking for a way to either "modify" my PayPal code or implement a new checkout system with my current PayPal shopping cart so when customers reach the checkout phase, the program will automatically calculate shipping based on order amount. I only ship USPS Priority, Express and International Priority so the amounts would be ...

EDITED: I figured out how to add the shipping preferences to my PayPal profile but you can only choose ONE profile at a time. So right now my settings are working for PRIORITY mail only.

From what I've read on their developer forums, if you want more than one shipping option for the customer to select from, you need to implement a 3rd party shopping cart.

So, what shopping cart systems are good when you don't want to rebuild your entire site; i.e., I'm happy with my custom order form and my site layout, etc. so I don't want to have to overhaul the site again with an ecommerce template.

I just want a new shopping cart/checkout system using the same order form but will give the customers the different shipping method options.

Today I had to invoice two customers extra for Express overnight shipping. They should have been able to choose that option and pay for it right in with their order amount. :-/

So, can someone point me in the right direction?

Thank you!!