eCart 5.0.1 was released last week to address those issues. here are the release notes:
* Changed the Update Cart server behavior so that non-numeric values used for Quantity will no longer remove the item from the cart.
* Updated the Add to Cart interface so that bindings properly retain their value after a change of focus.
* Corrected an issue that caused PayPal Express checkout pages and server behaviors to be added when 'No Gateway' was selected in the Checkout Wizard.
* Corrected an issue that caused an error to occur when attempting to activate eCart without an available internet connection.
* Corrected an issue that caused 'Your shopping cart' to be written twice by the Display Manager when using the 'Checkpoint' design with email and read-only display types.