This took me forever to figure out; because it was easy to overlook the cmd and upload requirement...
If you want to add multiple items to a PayPal shopping cart.. you MUST add the cmd and upload as below.
&cmd=_cart&upload=1&item_name_1=Registration%20Type%20-%20$(RegType_Text)&amount_1=$(RegType)&item_name_2=...&amount_2=....
You also must change your PayPal Payment Type to shopping cart.