help understanding record.save()

We use a hosting platform connected to Avalara to take orders and collect the correct tax amounts. Currently our integration pushes the tax rates calculated by Avalara to the sales order record to overwrite the default tax rates with the use tax rates we charged. Sometimes a user may need to edit the sales order and charge a different shipping tax rate. I've created a user event script to update that field when the shipping tax rate override field is changed and it will update the rate, but the tax amount does not recalculate. It won't calculate until the user