Don't give up..

[Scrumpy]

Marge says that it is very easy.
Even after you have sent an order, you can still go in to your account and back to 'your orders', click 'edit' and add items to it. 
After you have sent an order you will receive a message confirming it, and for several hours after receipt of the confirmation you can still add items, even if you have paid online.  I assume that you cannot add to the order if it has been sent for delivery, but they tell you the expected delivery date and time.

Thank you so much.. I will do as Marge instructed...
I do know that I am able to add items up to the day before delivery..