InmateAid does not touch your inmate's call card balance directly. The way it works is that InmateAid reduces what Securus charges for each call, so less money comes out of the account per call rather than InmateAid drawing from it separately.
Here is the math on your specific situation. Right now you are paying $1.36 to connect plus $0.66 per minute. On a 15-minute call that works out to roughly $11.26 per call. With an InmateAid number, Securus charges $0.21 per minute flat with no connection fee, so that same 15-minute call costs $3.15. That is a savings of about $8.00 on every single call.
The InmateAid monthly line fee is $19.95. At $8.00 saved per call, you break even after just three calls. Everything after that is money that stays in your inmate's account instead of going to Securus. If you talk daily, you are saving roughly $240 a month. Even if you talk a few times a week, the savings far exceed the line fee.
The reason InmateAid can do this is that Securus charges different rates depending on the type of number being dialed. InmateAid identifies the number that triggers the lowest available rate at your specific facility within the Securus system. Your inmate still uses the Securus phones, the call still runs through Securus, and the money still comes from their account. The only thing that changes is how much each call costs.
In your case the decision is straightforward. Set up the InmateAid line and your inmate starts saving on the very next call.