Whether you purchase your postage at the counter or at a self-service kiosk, you should receive a tracking number so long as you are purchasing Parcel postage.
If you select the First Class Large Envelope option instead of Parcel, you will not receive a tracking number, and tracking is required for all Kidizen orders. Please also keep in mind that while the item may fit in an envelope, the shipment may be subject to postage due if it is deemed a package, and not a flat (i.e. more than 3/4" thick), during transit. And buyers are not happy customers when they have to pay additional postage when receiving their order!
Note: If you are paying less that $2.60 for postage at the Post Office, this is one warning sign that you are not purchasing Parcel postage.