Hi,
Please confirm the software version you are using for us to advise and assist...
If you are using v5.00 and yet the issue persist on the same Mac where you have the application, download and install the diagnostic version, for us to gather more data of the case, click here > https://files.fm/u/mpae2gtd
Open the program,
Click the Ignore tab until the Express Invoice window open(you can use the "Don't show this message again" option).
Close the Trace_ExpressInvoice.txt window.
Go to the Web Access interface and reproduce the issue so the trace reflects the error.
When done, from the Help menu, select "Open Diagnostic trace" tab and highlight all the text to copy(Command + C) and paste (Command + V) on your email reply.
Best regards,