The BBIS cart should not be emptied/replaced if a user logs in.
If you add a donation/event registration to your BBIS cart, and then log into BBIS, your cart is replaced with the last cart you had while logged in. Because of this, it empties whatever you just added to the cart.
I would suggest that the cart combines what a user just added to the cart with the last cart associated with the logged in user.
I can't think of any other e-commerce system that removes the contents of a cart when you log in to the system.