In Martial Class™, sales, orders, supply of items, and payments are all recorded under (and referred to as) Transactions.
In any case, to sell something to someone, you need to get a Transaction Window open.
There are many ways to start a transaction, either by selecting "New Transaction..." from the right-click menu or the menu bar on the various windows that show students...
...or by drag-dropping a fee or item onto a student...
...alternatively, if you are selling to the public (ie a person not in your system) then you can also use the right-click menu on the Items Window.
Once you drag-drop an item either on a student (which opens a new Transaction Window) or on an open transaction window, you are presented with the Order Specification Dialog.
The description of the order specification dialog provides more instruction on the options available when this is displayed.
TIP: To change an ordered item size, you need to return/cancel the previous order item, and then raise a new order item.