@peachykeen159 We did just have the 30 days. And it was a huge rush! We basically called our guests that would be flying with the WestJet package out of Calgary and let them know that in order to get them the best deal we could find, they only had a few weeks to make a decision and put down their deposit. Everyone was really great about it, as most of them had already started to discuss whether or not they were going to make the trip. It was no surprise that we would be going to Mexico for the wedding.
I'm in the process of sending out invitations now, and our deposits were paid back in February. We have some family flying from other cities that are booking a hotel package through Sunwing and doing their own flights, since we wouldn't have enough people flying from the same city to do another WestJet package. Since there is no rush for them (there isn't a final date they need to pay by) I wasn't in a huge hurry to get the invitations sent. For those that did not want to make a decision right away, we sent them our travel agent's info and just let them know that the pricing was not guaranteed past the final date for deposits.
I think that if your friends and family already know that you plan on having a destination wedding, calling them and explaining the situation would be understandable. Just let them know a formal invitation will follow.