The current limitation is that tax can be set at a line item level only by assigning a product Tax Rate (setup in the Products settings section).
There is no way to put in a variable tax rate. There is also no way to create a Tax Rate record "on the fly" via API as it is UI entry only.
This creates a barrier to handling tax at a line item for complex tax jurisdictions like the US, where there are many 1000's of effective tax rates. It also blocks the use of best of breed third party tax engines such as Avalara AvaTax or TaxJar from being integrated for line item tax.
Current workarounds I see are using the quote level Tax to aggregate the taxes, or utilising custom properties at the line item level and customising the quote template.
@rncmarques There's a new app available, Sales Tax Powered by AvaTax, in the HubSpot Marketplace that will accomplish exactly this. My company built an integration between Avalara and HubSpot so companies had a way to automate sales tax calculation.
And leveraging AvaTax allows HubSpot to have assessed per line item while factoring in the many US taxing jurisdiction.
