A debit card works just fine. InmateAid accepts debit cards the same way it accepts credit cards, so nothing is stopping you from placing your order right now.
Head to InmateAid.com, set up your account, add your mom's facility information, and use your debit card at checkout the same way you would on any other website. The transaction processes normally and your letter goes out the same day if you order before 5 pm Eastern time.
Sending your mom a letter is one of the most meaningful things you can do for her right now. Hearing from family, especially a child, carries real weight for someone who is incarcerated. It reminds her that she is loved and that people are waiting for her on the outside.
Write what is in your heart. It does not need to be long or perfectly worded. Just let her know you are thinking about her. That is enough.