SheIn offers its SheIn Women's Cold Shoulder Keyhole Back Sheath Dress in Black for $7.90. Coupon code "SPR31" cuts it to $7.11. With free shipping, that's $3 off list and the lowest total price we could find. It's available in sizes XS to L.

Note: Uncheck "Add Shipping Insurance to your order" during checkout to remove the 99-cent fee.