I don’t charge for my art, for example. The reasons for it are simple: 1. I am a working adult and art is just a hobby to me. So I also don’t have time to draw that much. 2. I don’t like dealing with extra responsibilities to the paying customer during my free time, it’d stress me out. 3. My art is not so good 
Your art is beautiful and totally on comission level. I don’t see the slightest reason to be shy about charging for your work. However, before you start, I’d recommend checking out artists who have been doing comissions for some time. Look at their ‘terms of service’, so to say. Just to protect yourself from getting scammed (for instance, most artist take at least 50% payment upfront etc).