ACM - My most impactful purchase of $100 or less

I’m a longtime listener of the Tim Ferriss' podcast, The Tim Ferriss Show, also read some of his books, including his most recent Tribe of Mentors. Both in the podcast and the book, Tim asks people a lot of interesting questions, one of them is: What purchase of $100 or less has most positively impacted your life in the last six months (or in recent memory)?