The success of setting the correct cassette will depend on the applications you use. If the apps are set to use any cassette, rather than call a particular cassette, then making cassette 2 the default will succeed. But if the app calls cassette 1, then no amount of driver default settings will help. So in this case, you will have more success if you create another preset for printing to cassette 2, then you can ensure you select this preset when you want to print on plain paper. And of course you can select the envelope preset when you want to do this.
If you do want to look at modifying the printer defaults, this can be done via the CUPS web page.
To access CUPS, open Safari and type
http://127.0.0.1:631/printers
This will open the CUPS Printer administration page and will show all the printers currently configured on the Mac. For the printer that you wish to change the defaults for, click its name in the Queue Name column.
Clicking the printer name will open the printer administration page for that printer. Here you need to click on the Administration drop menu and then select Set Default Options.
This will show a new page with four menu headings (links); Options Installed, General, Banners and Policies. Select General. Scroll down the list until you find an entry like "Paper Source" or something similar that shows the cassettes and select Cassette 2.
With the option selected you then need to scroll down the list to the bottom of the page where you will see the ‘Set Default Options’ button. Click this button and you will then be presented with an authentication window. Enter the account name and password that has been set as the admin for this Mac. And then press the Log In button. You will then see another page stating that the information change is completed.
Now you can open an application and select to print and then check the appropriate paper source menu to see if cassette 2 is the default.