Is it tacky to email a birthday party invitation?

It is a 6th birthday party and it has crept up on me so fast. I want to mail invitations, but they wouldn’t go out until tomorrow and wouldn’t be received until the next day (Saturday). The party is one week from that day and I’ve requested RSVP by Wednesday.

I know I am already breaking rules of etiquette by sending them so late, but I don’t want to make it even worse.