It is potentially a gray area. Someone trying to minimize taxable income may interpret the tax laws to allow them to deduct the entire purchase so they will spend $15. Others will deduct only the minimum to meet the required purchase.
This is similar to when the minimum purchase costs more than the reimbursement plus fees (If any). What I do may or may not pass an audit but it is based on my understanding.
Two different tax advisors may have different understandings as well.
My posts are solely based on my opinions and for my entertainment, contact a professional if you need real advice.
When you get in debt you become a slave. - Andrew Jackson