Once you add a discount to a cart, it does not recalc or go away.  if you delete all products, the total will be negative, with just the discount showing.  This should be recalced with every addition / deletion / change in qty in the cart.

I'm testing the webstore using PaypalStandard and version 2.3.1.0 MySQL. 

Three peculiar behaviors -- I've added an item and offer. The item is a fulfillment type of 'none', the offer set up as visible and as a donation.  Logged in as a user with admin rights, I create a cart, and add the product to the cart, then click on the Paypal checkout button.  At Paypal, I decide to cancel and return to the portal, but instead of returning to the cart.aspx page, it attempts to return to a checkout.aspx page, where the credentials stored in the session string are refused and instead the AccessDenied.aspx page appears with the message "Sorry, you are not allowed to view this content. If you have questions about your permissions, please contact a site administrator.
Either you are not currently logged in, or you do not have access to this page within the site. Please contact the site administrator to obtain access. "  Of course, I am logged in with admin rights, so something else is amiss.
 

2nd peculiar behaviour -- similar sequence to the above, but I created a discount code and applied it to the cart.  The cart shows the reduced total, but when the cart is sent to paypal, the discount line is not sent -- so no discount is applied.

3rd peculiar behaiour -- if the item starts as a $25 item, and I have several discount codes established that could be used even when another discount is used, the total amount can be less than zero for the order.  It seems that a discount can be applied even when the total order is less than the 'minimum' for the discount, but an item is more than the minimum.