are you using Free Tools for this, or eCart?
if using Free Tools, use the Paypal add to cart button, and enter a price, then edit the code that gets added to the page, and change the rice element from a hidden fieldA:
type="hidden"
to a text field:
type="text"
this will allow the user to enter the price.
when they goto the paypal site, they will be able to add a shipping address during checkout.