Resolving additional payment options?

I'm trying to make UX on service purchase (it can be any product or service). I have a mandatory "Fixed price" field and I can add instalment options if needed. Here is a popup.

I need to make user enter a fixed price (this is more like a global price), then if he wants to, he can add an instalment plan. For example, I'm selling a mouse for 100$. I also want to add an instalment payment, 12 instalments, but for 110$ (do the math 110$/12=9.16, you get how much the instalment is).

After I added the option, it looks like this.

Help me figure out a better way of this process.