This situation can and does happen once in a while.  You process your order and then shortly afterwards you want to add something to your order.
Here's our best suggestion to this matter:

Purchase your additional item(s) as soon as possible after your first purchase and then call us as soon as possible at:  516-935-1110 Ext. 5, and usually if this all happens within 24 hours we will be able to combine the orders for you.