Please check the image!
PFA the image attached..
If you have your Project Security level set to “Production”
Then you’ll be able to use this microflow
Now it’s up to you where want to store the value of the total price
You can store it in already existing attributes or create a new one called Total for example
If the total attribute is in Cart entity, you can just add one more activity to the microflow above
Change object: Cart, Customer_Cart, Total, $SumPrice
If Total belongs to other entity you’ll need to Retrieve that entity object and then Change it to store $SumPrice in the Total attribute.