Issue with tax on donations(separate fee)

[mrsaicharan1]

Describe the bug
Whenever the tax is configured to be set as a separate fee on top of the price, due to the variable price of donations, the tax is pointing to a negative value. This is most likely due to the computed property which was written in general for paid & free tickets, without keeping donation tickets in mind.

To Reproduce
Steps to reproduce the behaviour:

1. Add tax and donation tickets to an event
2. Enter quantity & amount for donation
3. See error

Expected behaviour
Should show the correct tax

Screenshots

Additional context
Working on it.