@Jerome,
The problem only arises when an order (these are all books) exceeds the First Class international shipments maximums (at this point, I don't know if it is weight or, more likely, dimensions). I haven't seen it in any other context or situation.
For domestic shipments, we only offer media mail and priority shipping, neither one of which has a weight limit or dimension problem at this point.
To recreate it, simply visit our store at
www.nifdi.org/resources/store
, add several items to the cart and proceed to checkout, shipping to anywhere outside of the US (I've been using Iceland as my example because that is where this problem arose) and I expect you to see this problem.
In the screenshots below, you can see where I was in excess of some maximum (again, I don't know if it is weight or size -- I THINK it is weight and the 10# mark seems to be the cutoff for this problem; at least when using an Iceland shipping address) and got the error. By removing the second item (which was multiples, creating a lot of weight), removed the error.