You're correct, it didn't cost that much to ship. My initial estimate was based on a USPS medium priority flat-rate box which costs $17.10. I was able to package these carts in x 6"x6"x6" box with sufficient padding and get a custom USPS priority shipping rate from here (Tallahassee) to the buyer in West Virginia of $7.02.
Occasionally, for small items, where price must be kept especially low, or where tracking is not needed, I will use USPS ground advantage. Otherwise, I generally prefer USPS priority over other methods because it's usually fast, arrives with your regular mail, can be shipped to PO boxes and includes $100 of built-in insurance. Also, if the receiver uses the USPS informed delivery service, it will not only provide detailed tracking updates, but will also automatically notify the receiver when a shipping label for your address has been created.