Hey there! I've just delivered a IAP project in Unity last month. Check it out on my portfolio :)
I'm a senior Unity developer, been working on this for the last few years in a self-owned company. My specialty in Unity ar AR/VR mobile apps (you'll find some of this on my portfolio too).
I'm pretty new on freelancer, but I've a lot of experience and the best is I love what I do and I love doing it perfectly. So I'm confident that we'll work just fine together.
So i'm planning for you to create a kind of store/menu to purchase the items using a simple but professional design (let me know if you need a specialized or themed design for this matter). I'll set the IAP for you on the stores, test them and deliver just what you need.
I'm available on chat i you wish to discuss further details.
Best regards, from México