Shipping rate for subscription item with a mixed cart?

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Hey,

We have default shipping rate of 20US and free shipping over 50US - however if a customer orders both one off items and a renewing item in the same Shopify checkout and reaches free shipping - when the 1 renewing item renews is the default shipping rate charged or is it still free shipping for that renewing item?

When I test this in the checkout - it says free shipping with the renewal item for the first order but also that it will be free shipping the next month as well.

Any ideas how this works?