It seems there is a bug in saving product weights other than the default (kg, g). I changed my default configuration weights to be displayed in ounces (oz). When I try to add a new product, the unit is displayed as oz. However, once I save it and go back (to edit it), it is displayed in kg. Please see attached MS doc file with screenshots.
The shipping methods however does not have this problem (units are saved in oz). Therefore when I go to checkout, it must be trying to convert from kgs to ounces, and in most cases I am getting "The cart items combined weight is too big for all the possible shipping methods".