I think there may be a weight or item number limit... I ordered a ego aio the other day and included a few smaller items (drip tips etc.) and it did the same, once I knocked the extra items off I got the free shipping (managed to sneak one drip tip in with the order.
Looks like it's shipped with post.nl so I'm hoping they're not going to destroy the thing because of the built in battery which I've been told they've been doing recently!
this was the result of the free shipping promo for a $34 order. sent via dhl so just be aware of thisIn that case I'll try and order items separately and let you know how I get on