Shopify has a built in function for users to add HS Tariff Codes and Country of Origin directly in each product in their store. If you ship internationally it is highly recommended you do this, as many countries will reject parcels without the information.
Shopify has a detailed help article on adding HS Tariff Codes here.
In the Shipping section of each product in your Shopify store, there is a link to enter customs information which allows you to enter the Country of Origin and the HS Tariff Code.
They will then automatically come through with any new orders placed in your store AFTER you have added all the codes. Please do not add any decimals, just leave those out and add the whole number.
For existing orders in ReadyToship before you added the codes in Shopify, you will need to add them directly in ReadyToShip by following these steps.