From my experiences, they won't pay. This is probably not the only bill they aren't paying and they know you will have the least impact on their financial situation if they ignore you. The credit cards and financial institutions can ding their credit and get bill collectors.

On the bright side, you just learned a valuable lesson for what amounts to be very little money. You could have gotten stiffed on a major lawn or commecial account. From now on, make sure they pay in advance. If they want it every two weeks, have them pay for four mowings. Half way through the mowings, bill them again for the next four. You will always be one step ahead. If they stop paying, you stop the service.