Hi Seth,
Depending on the number of items you have, you may want to consider using a per item price level instead. This allows you to set a specific price for each item in your items list and will allow you to apply this price in both Method and QuickBooks.
If you only need to apply this logic in Method, you could add a custom field to your item table and add a specific value into this field for these items. Then check for the specific value in this field and if it’s found, apply a 10% discount instead.
There are certainly other ways to accomplish this, but this is the fist idea that came to my mind.
I hope this helps.
Audisho