Auto added products

You can create a plugin that overrides the UpdateShoppingCartItemAsync method.
However, instead you may want to try creating a plugin to handle EntityUpdatedEvent...

public class ShoppingCartItemUpdatedEventHandler : IConsumer<EntityUpdatedEvent<ShoppingCartItem>>
{

    public async Task HandleEventAsync(EntityUpdatedEvent<ShoppingCartItem> eventMessage)
    {
        // Your custom logic here 
    }
}