Hi, I have spoken to Ian about this and he agrees there was a bit of an omission in his catalogue this year. Simply, the situation is this, if he can send the autumn flowering ones early, then separate postage is required.
However, this year, he only has P. Barcena which can be sent early, so unless you are ordering P. Barcena, only single postage is required.
With his permission, I have added the relevant wording to his catalogue (link at the top of this page). I hope this clears it up.