Raise money for a math club

You're the advisor for the competitive math club and you want to get the students a good deal on shirts they can wear to competitions. Thing is, the deal doesn't last forever and you can't pay the money out of pocket yourself. You need to collect the money from the club members and you need to do it fast. You know they'll forget to bring the money in, so you need another solution. Something they can use to pay you immediately.