Even though Chiang Mai is old city but still modern and convenience; you can find many internet café that very cheap some place only 10 baht an hour, shipping is one of the convenience ways that you can send things home. Chiangmai provides you with many famous companies such as DHL, FedEx and UPS; you either go direct to their main company or agents, you will see lots of them at Night Bazaar, because the price is not much different.
If you want to send by post, you can do also. Thai Post will charge you according to the weight of the product but if you want to send big quantity and very heavy you should send by Sea Freight, they will charge you in cubic metre, if you prefer send by Air Freight all agents and Thai Post provide that service also but seem the most expensive way for shipping but very fast!
