I want people to be able to buy a product and then it will save so they won't have to buy it again and can continue having the things that the product sales after leaving, and after dying/respawning.
The whole point of Developer Products is to have something a person may buy that is stackable or buyable multiple time, such as ammo, in-game credits, speed increase by a certain amount, and so on. Of course, you could use Data Stores to save that they have bought it, but they still can buy I again, wasting their money.
I suggest you make a gamepass for the player to buy. This way you always have a reference on something the player owns, giving them whatever you want accomplished.