If you have at least 1 shiny Shellos in your collection (any colour), it will be a registered trade and cost 20,000 stardust and typically can be traded to you within 24 hours. 

Unregistered Shiny Trade 

If you do not have this shiny Pokémon in any form, it will cost 1 million stardust if you want to trade now. There is a second option where we can add each other as friends and exchange gifts for the next 30 days until we become Ultra friends. When we become Ultra friends, it will only cost 80,000 stardust.

Have a great day!