Have you tried phoning them first? I was in a job where I had to pay bills for an infantry unit, and certain small ones, like the cobblers (who serviced the boots for the band), I used to sit on. Not because I didn't want to pay them, just I had other priorities and it didn't seem important to me.
But one day the cobbler phoned me (he must've gotten my number from one of the band members), we had a chat, and all future payments I paid on time.
Just give them a bell and be polite and explain to them how your cash flow is important to you.