Short term payment plan questions

Hi, I have some questions on setting up a short term payment plan (180 days). 

- when you set it up, does it send you a letter in the mail stating that?  Or is the letter only sent for long term plan? 
- can you decide how much to pay each month, or do you have to commit to one amount each month?  Is it on autopay, or you do you in and pay manually?