This feature surprisingly does not exist!  You have two options

1) Create a 15% OFF discount and assign it to a button that needs to be pressed on each sale.  I don't recomend this method because, if you're busy that day, your staff will inevitably forget to press it on at least one sale.  And don't forget to delete it the day after so that it is not used without your authorization.

2) Export all of your items, use a spreadsheet program to apply a 15% price change, then import the items.  Ensure you keep the original CSV to re-import and change the prices back.  There are several thisngs to keep in mind.  For instance, my UPC-A codes start with a zero, so I need to change the format of that column to "text" then add the zeros back in.  Also, your customers will not "see" the discount on the receipt, it will have just charged them the lower price.  I also do not recomend this method lol.

Both methods suck, but it's what we have.