Here's a minimal checklist for your invitation (thread):
Chances are I have no idea where your institution is located. Don’t direct me to a website unless you are sure that website has an address on it.
How many people and what level of knowledge?
A list of dates, or at least a month or season would be of great help.
Does it take 4 hours on a bus to get there from the closest airport, the one where planes land only Tuesdays? Please tell me right away.
You probably had a reason to invite me, or did you?
This info is always almost missing. How the heck am I supposed to prepare a talk if I don’t even know how long it’s supposed to be?
I need to know before I prepare my slides, so please don’t inform me of this 5 minutes prior to the talk.
Do you expect me to also got to lunch with the students? Humor the dean? Sit on a panel discussion? Give interviews to journalists? Sign books? Attend dinner with the queen? I want to know about this before I agree to come.
Sorry for being materialistic, but before I book a flight I want to know who eats up my bills. And, yes, I actually expect to get paid if I work for you.
It isn’t always easy for me to work out my schedule and probably it isn’t any easier for you. So please let me know how I can help.