Sending items out on memo is similar to a Layaway Sale.
*) You still own the items
*) The items are not available for sale
*) The sale is not counted in the sales figures
*) when the sale is completed you finalize the sale.
*) Or When the sale is completed you Bill the sale
So in Shopkeeper you would enter memo sales using
option 1,2 Layaway Sales.
You can also go to Store Information Defaults - page 9
and change the receipt wording to reflect Memo.
If you later convert the sale to an in-house account sale
you can edit the sale change the sale type to "Charge"
If you do both Memo and Layaway then you could leave
the receipt wording for Layaways and use a pre-printed
receipt with Memo wording on it for the memos.