Custom Free samples products.

New Member
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Hi every one, 

I am new to Shopify.

I am create a site where i have planned to give free samples worth $10 to the customers who purchase greater than $80.
The Samples should picked from separate collection page.
If the samples product value greater than $10 excess amount need to add in total value.
So for i have done as shown in URL: 
My question is how to pass the total amount to checkout page. 

Kindly help me to get possible solution.