There is a Post Office located in Chaweng and Lamai.
Sure also at other places on the iland.
If you order something – sure often the Motorbike Post does not find your location, you can pick up your items at the post office.
Shippings from China (Lazada) to Koh Samui needed 3 days 😉
If you want to ship something – it is really cheap inside Thailands and nowdays fast (2 or 3 days to bangkok or other areas in Thailand.)
Also nowdays they offer SameDay Delivery.
Informations and prices at:
If you need something, you can not get on the iland – order it online and get it – you have to give your phonenumber with delivery adress (at every 7eleven you can buy cheap sim)
and they call you.
So do not worry , that your order arrives.