EXPLAIN FORMAT=JSON shows the cost the plain one hides

The tabular EXPLAIN says which index was used and not how expensive the optimiser thought each option was, which is the information you want when it chose one you disagree with.

EXPLAIN FORMAT=JSON
SELECT * FROM orders WHERE customer_id = 91 AND status = 'paid'G

-- "cost_info": { "read_cost": "1.00", "eval_cost": "0.20",
--                "prefix_cost": "1.20", "data_read_per_join": "4K" }

The costs are in an abstract unit and are only comparable with each other, which is enough — a plan chosen because it costs 1.2 against 4,800 explains itself. It also shows used_columns, which is how you confirm a covering index is actually covering. Verbose and worth reading when the plain form has stopped being informative.