The other piece of advice I would share is to see how many you have on the actual day or the day before & if you have space, say siblings are welcome to come along & that way you aren't paying for places/food/party bags that don't get used.
I'm with Alchemy on paperless post or a face book invite if you can but sometimes hard to get contact details for all the parents, if you include your mobile or email, some & I stress only some, may text or email you.
Good luck & have a lovely party