You are placing the attribute FinalTotalPrice in the Product entity. That means that for every product you create it is calculating the total price. In your screenshot I see 2 products. That's why there are 2 options to select. Try adding a third product. The dropdown then will show you 3 options, all with the same total ;)
The solution to your problem is a view entity. This view entity has an attribute ‘FinalTotalPrice’ and a association to the Product entity. The ‘FinalTotalPrice’ is calculated by adding the prices of all retrieved products. On the page you can show the products in a list and the total price in a textlabel.
Your Domain model should look a bit like this:
and your microflow prior to the page a bit like this:
Hope this helps!