I need help, customer service can’t count, and I am about to dispute the charges on my card that wasn’t deducted from my xfinity bill

I have a problem with billing, and everyone I have talked to at xfinity, I owed 275.00 last month on my bill, I made 3 payments, one was returned, my mistake, I used the wrong account, no money was deducted from my bill, or bank account, but I paid my bill on the 18th, 125.99, and on the 19th 137.99, which was 265.00, somehow they think I owed 137.99 more because my check was returned, and I owed 275.00 in November, not anymore then that, but I can’t get it pushed into anyone’s brain at customer service that they didn’t deduct 137.99, they think I paid back 137.99 from the returned check, which nothing was deducted in the first place, my bill was still 275.00, and I they just can’t do the math, I am getting extremely frustrated, it’s like they think I owed 402.99, because my bill is now 320.00, and I gave them almost all the money I owed them in November, except 10 dollars.