To initiate a return, you must inform us of your decision within the Cancellation Period by contacting Arduino's customer support and clearly stating your desire to return the product.
Arduino's Customer Service will provide you with a written return authorization ("RMA") that must be included in your return shipment. We are then able to identify your shipment and have a return address. The customer shall arrange to have the return shipped back to Arduino.
The conditions for accepted returns are:
Arduino accepts returns only within thirty (30) calendar days from the date of Customer’s receipt of merchandise. If you purchased the product from somewhere other than the Arduino Store, please contact that reseller. In case of defective products please refer to the “Warranty” section below. In case of return for withdrawal please refer to section 9 of the General Sales and Conditions.
You can review the full document here.