but do not forget to tell him that is never worth for a f2p to actually buy them with coupons because there are a ton of other features that have a way better 'useful-power/coupons-spent' ratio than refinements since is useless to have 15k initiative, injury and critical if you have 9k nin/atk/def/res and 18k hp...

@OP: if the reason why you want to buy advanced refinements is because you lack initiative, critical and injury, then a way better option for you to go for is to actually buy lucky key packs and the 88 cake key packs when available at 799-810 coupons (not at 900) and try to reach every time 1050 cave key used when the rebate is on. That gold essence tool + all the cave runes grant you way more of those stats than the equivalent quantity of coupons used to buy advanced refinements grant you.

consider to get 1 single level 10 you need 1000-1400 runes that even in the best case scenario (40% discount in treasure of the sage) cost you 22000-30000 coupons. With the same coupons you get 220-300 lucky key packs that are 2500-3300 cave keys to use in the rebate that mean 2-3 gold orbs + 40-60 rolls (that maybe grant you additional orbs) and 2800-3600 rolls in the cave exploration an indeed better investment.