We pretty much emailed everyone we would be inviting to the wedding in Mexico with all necessary travel information on there and then added that formal invitations would be sent later. We pretty much had a month from the time we booked our resort to time for people to put their deposits down. After that point there was no guarantee that the price would stay the same. Our flight was actually sold out quite quickly and they upgraded the plane and also released more seats (some must have been reserved for another group). So we chose the resort end of March and deposits were due April 21st. People can still book on their own or with TA but no guarantee they will be paying the same amount. Final payments are due October 31 and we fly out March 29, 2014.
We never sent out STDs as people obviously know when they are leaving if they booked their flights. I would have liked to but too late now! In the process of picking invitations now and would like to send them out by the end of September I would think (for the people who are still on the fence about coming they still have a month to decide).
Hope that helps!