I have been using the chest inventory for shops, and found that three slots have been left empty. So to fix this, the shop menu can have the pre-existing items sorted into 3 line of 8 which leaves a column for these new items. The middle of the column will have an item that cannot be bought that will have a description explaining the rotational items. The above and below slots will have rotational items. These items could be test items, or seasonal items. For the other shop layouts, they could get their own tab or fit into tabs that would make sense.
What about on new UI?