What was your best PAID marketing investment?

I used personalization tool name Twik.
It didn't cost much and the price method goes by your traffic.
The app itself allows you to:

1. BI data tool - manage your KPI's and learn about the user behavior. It rely on fingerprint and not cookies, which gives you a better understanding of your users.
2. User segmentation for marketing reason - want to target a specific audience? it's done. Just decide what's your conditions.
3. Personalization - create personalized experience you each individual user, and by that help increase conversions.

so far? nothing lol