Apologies for posting twice, I probably put this in the wrong place.
I have a client scenario where all products incur HST at check out. However, if certain products are purchased at or above a government defined minimum volume, HST is not chargeable on that item. I cannot figure out how to do that in the client’s online store.
Thank you
Ross Chevalier
Hey @Ross_Chevalier 
- this is a bit tricky for sure, but you are on the right track when it comes to using the Cart Transform Functions API. You’d likely want to look at building an update function: About the Cart Transform Function API
I can’t say for sure how your client is setting up the volume of the product (if they separate it volume amounts into separate products or just use the quantity of items purchased), but you would likely need to first run the input query we share in the docs there to grab the line item quantity amount/merchandise title to determine what product is being purchased and then run the update mutation to modify the price to account for the taxes being applied.
You’d need to check the cost.checkoutChargeAmount and the cost. totalAmount objects to grab the cost before and after tax in order to account for the HST though.
Hope this helps - let me know if I can clarify anything on our end here 
Thank you kindly Alan. This is a problem for my client, but not so large a problem as to fund custom development, that I am not qualified to deliver at this time. I sincerely appreciate your guidance on this topic.