OMG!! For a second I thought that I had written an entire post and didn't remember it. After I designed and made our own passport invites, FMIL constantly adds people to the list. But it gets better - she now says that we don't need to send invites because she just forwards our website as the invitation.
Sorry, didn't mean to hijack the thread but it's just identical to our situation. Try not to let it stress you out (I know easier said than done). Just try to explain that unfortunately you already purchases everything and can only make a certain # of invitations and would like to keep your guest list at a certain #. Besides, you sent the initial list months ago so it's not your fault she NOW wants to add people.
Good luck!!!