There is a large flat rate box called a "game board" box that the USPS does not stock in POs, but they will deliver 25 of them to your front door, free of charge. Order them online.
They are about $15.80 if I remember correctly. Otherwise, a keyboard the size and weight of a AT101, packed in a reasonably appropriate box, will probably ship ground for about $10-12 in-state or nearby, $12-14 for the neighboring region, and $14-16 to the other side of the country. If you use a box unnecessarily large, you will pay for it, regardless of weight.
Usually, the PO is the least expensive, especially as the distances get longer, with UPS and FedEx being more. FedEx is often the most expensive but has BY FAR the best tracking.