Amazon PPC, Freelance or Agency?

I’ve got a freelancer working on my PPC. So far, she’s gotten results after the 3rd month, and continues to grow my brand month over month.

However, I’m starting to wonder if what I’m paying for is best for value. I pay her $3k every month plus a % of PPC Sales on the campaigns she created given that our targets are met. In a month, I pay her from $4k to $7k, and this April, since performance was good, around $8k.

I was wondering if getting an agency could be better and value for money. Do you have experience with Amazon PPC agencies?

But don’t get me wrong, she’s getting results. Just wondering if I’m shelling out too much.