I got this working now under invoice however there is a cosmetic issue that needs fixed
When you select a taxable item it selects the correct tax however the dropdown tax menu doesnt auopopulate see here
For example Pudding Pops is selected has a 10% tax associated with it

When you hit submit it does apply the tax, it just doesnt update the select box

See commit here:
itflow-org/itflow3ec2e79