Nafl is not a replacement to sadaqa. Both have their benefit. And both are not meant to kind of buy the good-will of Allah. Let me explain:
Allah can help through the deeds of a person if he or she is faithful and accepts His guidance. If someone has enough ressources and decides to share more than his due zakat, he will help others who do not have enough not to get into trouble. So it will postpone or remove the adversity from the one who receives. The one who gave sadaqa may profit from it seeing the peoples relief, or that nobody would rob him or her.
Alike, you can do good and let Allah help through your deeds if you are faithful and accept His guidance. Your contribution will be immaterial, to be helpful and kind to others. If you add Nafl, you should do that if you want to increase relation with Allah. If you pray, Allah can give you the strength to be fine in the situation you are. And you will probably think and do something of His Will.
Many people say that if you do so-and-so God will do so-and-so to you. This may sometimes encourage people to do so-and-so and that is good. However, it does not usually mean that Allah now takes action to recompensate and do good directly to this individual. Rather, He may act with others, and in a whole, the good will increase and finally come back to that person.
Because God is One for all people, animals, plants and non-living nature. And through any of it, God will increase good, and on any of it, we can be part of His action.
So whatever you do in true faith what is the Will of Allah, it will be good, and the good may come back to you.