This is the normal behavior with Payments Standard. There are no features or options to change or alter the way process works. It has nothing to do with the standard item button code.
So what are the options. One option would be to use a 3rd party shopping cart. Another option would be to develop your own mini-cart if you're into programming. Both options use PayPal's "upload" method whereas only the final results are passed to the PayPal Checkout Screens. Many 3rd party carts run a backend database which stores the selections, manages inventory, contains shipping tables, etc. and allows you to view a cart stored on your web page or site. Mini-carts are not as sophisticated but function is a similar manor. Both only send the customer to the PayPal Checkout Screens one time instead of each time you add an item to the cart.
You can learn more about the "upload" method and it's specific HTML variables in this desktop guide.
https://www.paypalobjects.com/webstatic/en_US/developer/docs/pdf/pp_websitepaymentsstandard_integrat...
Want to learn more about mini-carts, see this link:
http://ereimer.net/nopercart.htm
As for 3 party carts, check out this one, very easy to use:
http://www.e-junkie.com/
More can be found here:
https://www.paypal-marketing.com/emarketing/partner/directory/directory.page;jsessionid=BE3436438EB8...
And there's a nice one here:
http://www.coffeecup.com/shopping-cart-pro/
(If you try this one, test the free trial first, else if you purchase first and change mind, there's no refunds.)
