If it was only a month ago, you still have time left because FrontLine TopSpot cat flea treatment is good for
six weeks, not a month, so if you use the half dose on each of them, they should both easily be protected for another 30 days until you can afford to purchase another 3-pack. OR, if you think you will be able to afford another 3-pack in the next two to four weeks, just hang on to that single dose you have and you'll have to FULL doses when you buy the three-pack.