How much coupons you have? I would wait and use it for lucky board since your power level can still be higher and lucky board helps out getting charms and refines. Players have stated that people should get clothing when they have power at least 60k. If you don't spend any cave keys for the entire month and wait for rebate, you can get close to at least 100 keys just by getting it freely via events so I would rather save the coupons if I know i'm going to get that much in the month.