Add a pourcentage discount on the base price.

It would also help if these price breaks were able to be visually shown to the customer. Something similar to this.

Adding bulk pricing for extra prices vs only base price

For my shop it is important that the discount be reflected for surcharges (in our case embroidery) as well. Gregoire's solution of setting a % on the total price is fine, or it would work for us if we can set a fixed price for each surcharge at the various tiers we configure.