Invitation curling will sometimes happen on printers with paper paths that are not straight through. Many of the printer manufacturers have such models. If curling does occur, one suggestion is to take a stack of the curled invitations, and gently bend the stack back and forth. Generally speaking, this will take the curl out of the cards. If you do have access to a straight path printer, such as Canon, Epson, or Lexmark, I suggest that you use them as a first choice.