How to charge a service charge

I have a client who wants to use Manager in a small guest house. They charge a service charge of 10% of the total invoice before GST and GST is also charged for this 10%. How would I go about doing this without manually calculating the service charge every time and putting it to a new line?

Example:

1 room = 100
service charge 10% = 10
Tax 12% = 13.2

Total = 123.2

I don’t think there is a way.

To be honest, if service charge is 10% then it’s fairly trivial to add new line item with the amount which will be 1/10th of the total so far.

Just make sure when invoice is created, the option Amounts are tax inclusive is unchecked so tax is added on top of the total.

That is true. I just wondered if it could be done through the software so that they don’t have to think about it.