Add-ons are always multiplied with Cost, even when not selected

[Valentina R]

If the 'multiply cost with quantity' is not selected, it multiplies by cost. If it is selected, it multiplies by 2 and by cost.

Can be tested here.

Reported here: https://bookingwp.com/forums/topic/child-option-not-working-properly/

[Željan Topić]

Fixed with 4.2.0