The interface is a bit confusing, in that when you pre-purchase it also asks you to select a subscription plan. I already have recurring Plus, so was worried about leaving that set to No Plan. I did so on the theory I'd rather re-start Plus than undo a double billing.
No worries, checking my account after ordering Summerset shows I still am an active recurring Plus member. So go ahead and ignore the Plan part of the page when hitting Continue on your purchase.