The link to pay your water/sewer bill online is wtmapa.authoritypay.com.
You can pay it with a one time payment from your checking or savings account with no fee (make sure to process the payment by the 23rd so that you don’t miss the penalty deadline – it takes 2 days to process your payments).
You can use your credit card to pay at this site as well. There is a fee of $3 or 2.65% (whichever is higher) to use your credit card.