Child support

I keep getting letters about an offset basically my ex is approximately $4500 behind in child support. I got the same letter twice earlier in the year and one yesterday followed by an additional letter asking to confirm my address. I moved out of state a year ago, haven't heard from him since 2015 and he rarely pays so I usually ignore those letters. Has anyone actually gotten a letter and received the back pay? Football season is over now my son wants to learn how to swim which is not gon be cheap. All this stuff I have to pay for I have no problem doing it this is what he needs but after all these years financial help would surely be a relief. Football alone ran me about $500 and that's just equipment/gear.

Update: why is it when you ask a question on this app you people get in your feelings and go into attack mode? Yes my kid does need swimming lessons it's called hobbies he wants to do it so why not! I can pay for it, I have been paying for it, I'm used to paying for everything I been doing it since the day he was born but I don't think there's a single parent out there who'd refuse the help. Yes I usually ignore the letters but I still open them read the first two sentences and toss it until I got the same letter a third time none of my family or friends can tell me what it means so I'm here asking a question. He's paid a total of $40 and I never got the money so I don't know how this goes and he put himself on child support to get visitation which he was granted until broke it for whatever reason. Hope that clarifies everything now can someone please answer my question?

Update: Maria & Sara who pissed you guys cereal this morning like seriously what was the point you two obviously didn't read. Stacking his money how the hell am I doing that when I clearly said I haven't received any money in four years? I created an account that shows me when he makes payments, how many payments his made, etc. His last payment was in 02/2015 and he has an outstanding balance. I'm just trying to understand this I'm currently at work. I been about this mommy life for 10 years this is what I do. 😎

Last update now Maria you sound ignorant no he hasn't made a payment but I'm still getting these letters. If there is a possibility I could get the child support money my son deserves regardless of his dad's presence in his life why not take it? Maria you're problem sounds personal you're not hurting my feelings bro move it along you sound stupid telling me what MY kid doesn't need. Thank you Hannah I wish I could call but I work 5 days a week and I usually get off after business hours. I tried calling yesterday I was on hold for my entire lunch break.