I want to sent a package to kathmandu, nepal from Kunming, China. 20kg, 110cm x 60cm x 45cm.

I'm afraid parcel of this size cannot be delivered. You should pack the item in a smaller box whose maximum length and circumference add up to less than 3 meters.
The cheapest way is surface mail and it costs CNY250 for a 20kg parcel. The delivery should take around 1 month.