Allow clients to change or remove coupon

[marlena-b]

Currently, clients cannot modify or remove a coupon once it has been applied to an order.

For example, if a client applies a 5% discount coupon and then finds a better coupon offering 10% off, they receive an error message stating "Coupon already used!" and cannot switch to the better discount.

We want to allow clients to:

• remove an already applied coupon from their order
• replace an existing coupon with a different one