I have been investigating the reported bug forge.joomlapolis.com/issues/8485 in your other thread www.joomlapolis.com/forum/255-developer-members-support/243888-subscription-quandary?start=0#324571 , and while there is a bug when mixing amount and percentage in the same tax rate, that implementation doesn't address your use case.

To address your use case, you can use Tax priorities as follows (I am taking as example the use-case of your first post in this thread www.joomlapolis.com/forum/255-developer-members-support/243869-solved-taxes-help#324485

A. Create a tax rule for your special partly taxed items
B. Create 4 tax rates, assigned to your tax rules:
1. Priority 1: remove fixed amount of 75 (fixed amount of -75)
2. Priority 2 (ordering: first of priority 2 for that tax rule): add federal tax of 8%
3. Priority 2 (if the local tax applies on same amount as federal tax, otherwise choose different priority): province tax
4. Priority 3: Add back the removed 75 (fixed amount of 75)

Of course, for step 3, have one tax rule per province.

Here a screen-shot that shows the displayed basket result:



I will look into your other thread separately, and reply there too.

I have implemented a new tax-rate kind implementing in a single tax-rate the above (actually 2, one for federal tax and one for province tax, instead of 4 like above), which will be available in the next CBSubs nightly release, pending full test completion.

However while with the new tax-rate kind, the tax amount appears correct as a result, it doesn't show details of computation on the invoice like above with 34 tax-rates, so might not be compliant with local tax-regulations on taxes statements on invoices. We always recommend checking invoices tax-regulations-compliance with an account.

Ok, the CBSubs nightly is now released. You will find a new tax rate type:

"A percentage R on the item price P to which an amount A is first added/substracted (R*(P+A))"

That corresponds to your feature request.

The other tax rate types have been better documented too in the drop-down, and with the corresponding formulas added to description.

forge.joomlapolis.com/issues/8513