For a one-time insurance or admin fee, create a product and add it manually to the student’s next invoice — not as a subscription package.
More → Products → +. Enter name and price (e.g. “Insurance”, CHF 120) and save. The product is then ready in the list.
Open the customer → Further information → Next invoice. You see all open lines before you finalize the invoice.
+ New line → Select from product list → tap your product, check quantity, save. The line appears on the next invoice.
Tip: Subscriptions and vouchers are for lesson packages. One-time fees belong on the invoice as a product. Name it the way you show it to the student (insurance, admin fee, …).