When it comes to eCommerce, you won't be able to add all the functionality you're seeking without some substantial scripting.
Most quality carts offer the features you want, but as you say, they're often complicated to integrate with an existing site. That's the nature of the beast though and you're going to have to make a decision to take the plunge and convert your site or substantially rework an available ecommerce solution.
I'd recommend vpasp. For a couple of hundred bucks, you can have your cake and eat it to. They've got integrated features that'll allow you to manage content as well as offer customer based pricing, coupons and incentives.
You can continue to use PayPal as your payment system or move up to one of the hundreds of gateways they support.
Realtime shipping lookups, unlimited categories and products, email confirmations, saved carts, recommend to a friend, you name it, they've got it.
If you just want to get started, you can download a functional but limited free version of their cart. Build your site template and you'll be 1/2 way there. Once you're ready, you can enable the extra features with an option pack.
It sounds a lot tougher than it is but it can be modified or, if you prefer, they do offer expert setup for a nominal fee.
Shopping Cart Software – Ecommerce Solutions and Web Hosting From VP-ASP