You get most power from training potions,refine runes,rune stones,so focus heavy on these.You can spend every 2 weeks around 2k coupons to get 100 flag fragments,to unlock all cards in assist ninja training,where yo use training potions.Refine runes,you know the deal-groceries,sage,space-time shop,monthly events.For rune stones,just save 1050 or 2100 cave keys,and wait for cave rebate.

So,you don't need to spend all your coupons,to power up.Best is to save as much you can,then spend when you have 25-30k and fuku deal is on.