I discovered that PricesIncludeTax dooes not trigger the IsTaxTotalUpToDate flag to change on the serializable carts. Either the marketId or currency needs to be changed to trigger that flag to change.

My new question is this: Is there a less hacky way to do this? It would be nice if epi offered a cleaner solution for manually forcing shipping or tax recalculations.

I had did this in a similar situation in the OrderGroupCalculator, taxes getting calculated in an external system:

public new OrderGroupTotals GetOrderGroupTotals(IOrderGroup orderGroup)
        {
            if (orderGroup is IOrderGroupCalculatedAmount calculatedAmount)
            {
                calculatedAmount.IsTaxTotalUpToDate = false;
            }

            return base.GetOrderGroupTotals(orderGroup);
        }

Thanks Jeroen. I tried adding the above method to my OrderGroupCalculator but it never gets called. Then I tried adding the below code to the end of the cart validate functionality on the shopping cart page and it appears to work. (I also neeeded to reset the IsShippingCostUpToDate.)

I am a bit concerned about using this approach however, based on Viet Anh's resonses in this thread about that possibly being inconsistant. https://world.episerver.com/forum/developer-forum/Commerce/Thread-Container/2018/5/recalculate-tax/

if (orderGroup is IOrderGroupCalculatedAmount calculatedAmount)
            {
                calculatedAmount.IsTaxTotalUpToDate = false;
            }

            foreach (var shipment in orderGroup.Forms.SelectMany(x => x.Shipments))
            {
                if (shipment is IShipmentCalculatedAmount shipmentAmount)
                {
                    shipmentAmount.IsShippingCostUpToDate = false;
                }
            }

 
We can try to calculate tax for shipments using IShippingCalculator with CalculateShippingTax + CalculateSalesTax, I think using this approach we can bypass the IsShippingTaxUpToDate flag.