Letters sent through InmateAid travel via the US Postal Service from InmateAid's Florida office. Transit to the facility typically takes 2 to 3 business days. After the letter arrives, the mailroom staff opens and inspects it before distributing it at mail call, which adds more time. Plan for the full journey from send to in-hand to be somewhere between 5 and 10 days, depending on the facility.
For writing back, all your inmate needs is a postage stamp, paper, and a pen. They do not need to use any service or pay any fee to write a letter back to InmateAid's address. They simply address it to InmateAid in South Florida, put a stamp on it, and mail it. When it arrives at InmateAid's office, it gets scanned and posted to your account for you to read. You pay a small fee to access that reply on your end, but nothing comes from your inmate's funds for the actual letter they send.
If you want to put money on your inmate's commissary books for stamps and general use, find the approved deposit method for their facility through InmateAid's facility page. Most facilities use services like JPay or Access Corrections, and you can deposit funds online using a credit or prepaid card. Having a small amount on their books means they can always buy stamps from commissary and write as often as they want.