Dear Mews, 

I noticed that on the billing screen, an added product is henceforth displayed twice.
(although in fact consumed only once)

I see this is to do with either “Linking product to reservation” or not.
If you do (which is default), the 1 product is listed twice on the billing screen.
If you don’t, the 1 product is listed once on the billing screen.

1 product being listed twice on the same bill creates confusion - it gives the idea that there were two consumptions (whereas only one was taken).
However, as the default setting is to link the product to a reservation, there must be a good reason to work this way? 

Please let me know what the best way to work is.

Most clear way to work, as far as I am concerned, is that 1 added product is listed once on the billing screen (~ siutation 2).

Any implications I should be aware of when not linking a product to a reservation?

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I will visualise both situations: 

Situation 1: one product (minibar consumption) is added and ‘product is linked to reservation’ (default)
→ displayed twice

Situation 2: one product (minibar consumption) is added and product is not linked to reservation
→ displayed once