Here is the definition of 'stiff' used as a verb: http://learnersdictionary.com/definition/stiff [+ object] US, informal : to not give as much money as you should give to (someone) He stiffed the waiter on the tip. [=he didn't give the waiter a tip or he gave a tip that was too small] In your example, the speaker wasn't stiffed in terms of money but in the incompleteness of the invitation.

In this context, 'she was stiffed' means 'she was cheated'. This would be correct only if she was expecting or promised something more than that.