what paypal system are you using?
Using paypal standard, you wont be able to do this but using payments pro, you could set a session on the paypal checkout page and add a Charge rule to the eCart object that triggers off the session value.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.