You cannot, it must be done before the customer enters the Check Out process.
At the present time, PayPal's Website Payments Standard does not have any built in features or options available to do what you have in mind.
There are a few work arounds.
The first is to use the "discount" variables and a some custom scripting with your individual item button code. You can then offer discounts, coupons or promo codes. See the Tips and Examples here for ideas.
Second option, you can built or have developed a custom mini-cart cart application where as you can apply a discount or coupon calculation on your web page or site first, then end results are uploaded to PayPal.
Third option, go with a 3rd party shopping cart that has the features you have in mind.
Regards,
skier
