Another soultion is:
It might be something massed up with his preference plist file.
Go to /Users/USER_NAME/Library/Preferences/ByHost/ and open com.NCH Software.ExpressInvoice.001b63abeadd.plist file. the number 001b63abeadd might be different on each mac.
Then, change InvoiceWindow/Height to , for example, 564.
Or, you can just simply delete this file and restart ExpressInvoice to force it regenerate the file with all initial values.