Sure, you can go with store-bought, but consider these personalized ideas instead:
Order them premade from a party website for $1 to $3 each. Try Cardstore.com (they'll address, stamp, and mail them for you) or Birthdayinabox.com (which offers invites that coordinate with their theme partyware). Factor in shipping time so you're not scrambling.
Send e-mail invites. Some sites, such as Evite.com, also track responses for you.
Whatever you choose...
Include this info:
* Invitees (Should a parent plan to stay? Are siblings invited?)
* Ending time
* RSVP deadline (but be prepared to follow up with stragglers).
Let your child add postage stamps, stickers, or a little drawing to the envelopes.
Send out invitations at least two weeks before to make sure your party gets on people's calendars (but not too early, or it will lose the anticipation factor for the kids). Unless you're inviting the entire class, mail invitations to save hurt feelings.
Think of a few well-priced gift ideas for your child, in case a parent asks what he'd like. Books and art supplies are a good bet.