We all buyed the game on steam or ealier for a small amount of money. But maybe some of us want to spend some more money on the development, i think every euro/dollar counts, cause it makes sense that you dont have to focus on creating 3d assets yourself. You need more money in some state of development so i would suggest to offer an donation system where we can spend a little bit.
Important no one forces others to spend and as donator you will get nothing additionally to the game itself, its just for those who want support freely!