A subscriber asks for one extra item in their next delivery, without it coming back every time. This is what Subi can do today.
From the Contracts page
You cannot add a one-time item to a subscription yourself. Products you add from the Product details window (the Select products button) join the subscription and are billed with every order after that.
From the customer portal
Subscribers can add a one-time item to their next order themselves, through the add-ons you offer with an upsell rule. When the rule allows a one-time purchase, the item shows as "Available only in the next order" by default and is not billed again after that order. Setting this up is covered in How Subi Upsells Work.
While a one-time item is waiting
Subi may answer an edit with: "Editing is temporarily locked because a one-time item was added to your upcoming order. Once this order is processed, you'll regain full access."
Until that order is processed, changes to the products, prices and quantities of this subscription are blocked. Cancelling still goes through.
The portal has no control to remove a one-time item once it is added.