The customer enters codes in the cart and during the checkout. You can hide that form in the component options under Show promotion code form if you only work with promotions that apply automatically.

An entered code stays with the cart even while it does not qualify. If a customer applies a code for orders above 50 EUR and then removes an image, the discount disappears but the code stays, and the discount comes back as soon as the cart is above the threshold again. The checkout lists such codes together with the reason they are currently inactive.