Well, I can throw some JavaScript at you. The file may be overkill, and what you really need is some simple stuff just to address your needs. But at least this servs as an example of what can be done...
http://members.aol.com/paypalhelper/auto.html
If your requirement is as simple as you state with multiple items, but each item has similar requirements, then I could see it being done for about 15 lines of JavaScript and minor changes to your HTML. It would be about this level...
http://members.aol.com/paypalhelper/simp1.html
You talk about a cart, but you posted this under single item purchases. It's a lot simpler for SIP than for a cart, but the general approach is the same.
Ron.
PS - you cannot do arithmetic in HTML - - it requires script either client-side or within a server.