At this point, we don't have a way of "greying out" an item, showing a countdown of stock, or automatically removing the item when depleted.
However, it's quick and easy to remove it from your menu and place it back on when it comes available again.
Essentially, it's the same as removing and replacing items on your normal iPad menu. Head to your management site and head to Regulr > Menus and remove the buttons you need to by dragging them off the menu preview. Then hit Save.
When the items are back in stock, Pop them back on to the menu and Save again.
The changes will come through to the Regulr app as it's loaded up but if you’re in the menu on the phone already you’ll need to refresh.