Problem: When updating a invoice created under the old Quickbooks 2006 (converted on April 30, 2009) a problem may arrise. If the qty or rate fields are changed, all the amount values change to weird values, and the PST and GST become zero.

Solution:

When updating one of these invoices, make sure to change the tax code to something else and then back to the intended code (ex. change from S to G and back to S). This needs to be done only for the line item being changed. All the values should calculate fine.