Previously, weight based shipping methods accepted an upper charge
field. When no weight band matched the baskets weight, this was
returned. I am not aware of a single retailer who calculates shipping
charges like that. It's impossible to pick a sensible value for a
retailer, as there's no upper limit to the amount of items ordered, and
any shipping charges would have to cover costs for that.

Seems ValidationError got a rewrite for 1.6.

Based on http://stackoverflow.com/questions/12384460/allow-only-positive-decimal-numbers

Hmmm, Oscar has a PositiveInteger model field but I suspect we don't
really need it (or should refactor it to use validators).