It happened today. It wasn't my brides fault, she ordered plenty to begin with, but moms decided that there were more people who absolutley must get invited.
Always order more invitations than you think you need. At least 25. Why? Because chances are that you will forget someone, a few will get lost, a few will get ruined, and your moms will come up with at least 10 more people you have to invite. Adding 25 to 50 invitations on to your order should only slightly increase your costs. Doing another order entirely, after the fact- will cost you almost as much as your orginal order.
So that you can see the difference:
Original order (100): $185
Original order plus "just in case 50": $281
Original 100, and then new order of 50: $333
That's a $52 difference (or 8 pina colada's on your honeymoon).
