Yes, you can set it up so that your inmate’s reply comes back to you online, but it is not a traditional “prepay” like paying for their postage.
What you are doing is using the InmateAid Response Mail service.
Here is how it works:
The cost, usually about $1.49, is charged to you when the reply is received and processed. The inmate does not pay anything extra beyond a regular stamp, unless they are indigent and provided supplies.
So in simple terms:
This option is popular because it keeps your home address private and lets you read responses quickly from your account.