The hair extensions can be shipped by regular mail or by using a courier firm. Regular mail is cheaper on short distance, but make sure you sent by registered post. Although this is more expensive, it will ensure you the hair extensions will arrive at your client address and if something goes wrong you will be able to investigate where it went wrong.
If you sell abroad, in most cases shipment by courier is much faster. You should be able to get discount on your shipments, in most cases depending on the estimated number of shipments you can get a discount on the regular price.
Check if you ship to other countries, especially when these countries are located outside your economic zone what the customs regulation is. If on entrance a fee is applicable, make sure you and the client have agreed upfront who will pay these fees.
As bad things happen, it is possible your shipment will not arrive the final destination, for example when there is a misunderstanding on the address, or your client is not always at home. For these cases make sure the phone number of your client is visible on the parcel. In this case the local mail or courier can more easier contact your client to ensure proper delivery.