I have been having issues with my board as well lately with payments saying "This Invoice has already been paid. for more information, please contact the merchant' via Paypal.
I believe the problem is because when you select the payment method via the drop down box and green box appears at the top of the page, if a user clicks 'confirm my order' before this has finished they get errors.
Also for the drop down boxes that have 'yes' or 'no' for example, can we have a default option that appears automatically.
So for example on my site which sells skins, we have 2 custom drop down boxes. One says 'add branding free' the other 'add installation', having these options automatically say 'no' would save confusion for customers.