Select the Sites tab from the menu on the left-hand side.

Select Unit Types. This will provide you with a list of all unit types in your storage site.

Select "..." and Edit Unit Type next to the Unit Type you wish to add a Promotion to.

Scroll down to 'Enter Prices'. You can then select the discount from a drop-down menu showing all the 'Storefront Promotions' you have created.

When you are finished select 'Update Unit Type' at the bottom of the page.

You will then be able to see the discount next to the unit type on your Storefront. You can add discounts to as many unit types as you wish.