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The option for Envelope #10 should appear in the Print Preview under Paper Size -- Does that option not show up or does it show up and it gives you an error?
The print preview window is within Google Docs
Take a look here: http://www.techrepublic.com/article/pro-tip-how-to-create-an-envelope-in-google-docs/
Are you using an Envelope template? Take a look at the steps on the link above
In the print paper size, do you see an option for Envelope?
The previous PNG file only shows the selected option -- If you click on the Paper size option where it says "Letter" then it will show a list of options.
After your last reply, I wanted to do some research on the issue and this is a very common issue lately and at this time, there is no solution to the problem that has worked in the forum.
I understand that you are upset but if Google has not resolved an issue with their website/program then there is nothing that can be done until there is a fix for it.