Shipping is calculated on the total legal currency collected and not on the total price of items.

If a $55 item is purchased with $10 in CC Cash the total amount of legal tender collected is $40.00 so then you would still be charged for the shipping.
Again, so long as the total amount of collected pre-tax money is over $50 the shipping is free.
This is one of the trade offs for having a CC Member store and the amazing deal that is CC Cash!