More flexibility for extra services

Please make it possible to have a service with extras so that the service's minimal duration and price can be 0. Let's take a cleaning service, for example.

The service is chairs cleaning. We want clients to select how many chairs they have that need cleaning (can be up to 100).

Therefore we have found the best and only way today to use the extras option, where the client selects the quantity. Meaning, according to how many chairs they select, the total duration and cost will be multiplied by the quantity.

The problem is that with this setup:

a) the extras' duration can't be greater that the service's, but it doesn't make sense, because the cost will be only based on the quantity selected and I would like to set the chairs cleaning service base fee 0 and duration 0 min, to which the selected quantity x duration per one chair will be added;

b) in total, the service's duration (which I can't put as 0 min) will be added to the quantity's duration, but again doesn't make sense, because the total appointment duration should be based only on the quantity that the customer selects via the extras (i.e how many chairs there are to clean). And not have anything added to it (even 1 min, which currently is the minimum option and messes up the calendar).