Threat of losing "discount" if I do not

Why am I being asked to use a debit card or bank account to “keep my autopay discount”? I have been with T-Mobile for many years and when we signed up for the seniors plan the price was $30 per line complete and was told the price would not go up. It did not matter if I sent a check for payment or used my credit card. There was no discount here. That was the price. Sending me this notice is bull and there are a lot of companies courting our business with lower prices.