When an order is a free order, order_total parameter is absent in the json request causing API failure on MailChimp's side.
{ "id": "39", "customer": { "id": "2118168afa0cc87ad0186b3fc2690878", "email_address": "...", "opt_in_status": true, "first_name": "Tye", "last_name": "Test", "orders_count": 6, "total_spent": 0, "address": { "name": "billing", "address1": "123 Ice Cream Street", "city": "Atlanta", "province": "GA", "postal_code": "30303", "country": "US", "phone": "404-444-4444" } }, "financial_status": "paid", "fulfillment_status": "completed", "currency_code": "USD", "processed_at_foreign": "2016-10-11 23:48:54", "shipping_address": { "name": "shipping", "address1": "123 Ice Cream Street", "city": "Atlanta", "province": "GA", "postal_code": "30303", "country": "US" }, "billing_address": { "name": "billing", "address1": "123 Ice Cream Street", "city": "Atlanta", "province": "GA", "postal_code": "30303", "country": "US", "phone": "404-444-4444" }, "lines": [ { "id": "9", "product_id": "10", "product_variant_id": "10", "quantity": 3, "price": "0" } ] }
{ "errors": [ { "field": "", "message": "Required fields were not provided: order_total" } ]