Are they able to write back on here

Yes, the service is called "Letters from Inmates". It allows the inmate to use the InmateAid address to receive your mail without giving out your physical address. There is a section on your Account Dashboard where incoming inmate mail is placed for your review. This is a huge service for people not wanting to expose their physical address where a bad-intentioned inmate could potentially extort a loved one on the outside. For $1.59, it's inexpensive 'peace of mind'.