Discount coupons have specific conditions that affect certain products. If when you apply it you don't see it reflected in the total of your purchase, it is possible that the products you have selected are not included in the promotion.
The specific conditions for each coupon are communicated in the promotional email.
Normally, products excluded from any coupon promotion are: BUFF® Filter Tube, BUFF® Filter Mask, Filter packs, BUFF® Customized, clothing and accessories, products in the outlet section and products that already have a discount applied to the web price.