Raspberry / Peach extract flavoring for White Grape wine

Wally World was out of Welchs White Grape / Peach concentrate and juice today so I ended up buying the white grape concentrate. I have 2oz of Raspberry Extract (Austin Homebrew) and 4 oz of Peach Extract (Northern Brewer) available to use as flavorings. I will be making 2 gal batches.

I am looking for the following suggestions

1. Should I add it at the beginning of fermentation or at bottling
2. About how much should be used? Like to know what others have done from a personal experience.

I made a 20%ABV Welchs Peach - White Grape before and everybody seemed to thing it was the best out of 15 flavors.

I have never used extracts though. I would get 5 or 6 pints or quarts of raspberries to smash up and add to the fermenter instead.