A bit of an overview and value comparison in this rebate

25 milestone = 1 charm pack, 3 mood scroll

50 milestone = 2 charm packs, 1 stamina potion

150 milestone = 3 charm packs, lvl 4 magatama

250 milestone = 5 charm packs, lvl 5 magatama

500 milestone = 10 charm packs, lvl 5 rainbow magatama

1000 milestone = 8 charm packs, lvl 6 rainbow magatama

By looking only at the charm packs the most optimal way would be doing 250 rebate each time

since you get a total of 11 charm packs by doing 250, which means it's 22 charm packs in total if you do it 2 times (compared to 21 total packs by doing 500 rebate)

And indeed if you do not buy any charms and don't get many of them throught the week doing 250 each time would be ideal

However this method is quite slow when it comes to newer players, with buying just a few charm packs you could probably easily reach 500 rebate per month to make your charm progress faster

Now this also depends heavily on how much the players spends, for big spenders doing 1000 each time wouldn't be a big problem I believe

But I wouldn't prioritize charms so much, it's probably more worth to just do 500 each time still if you aren't in rush (or if you don't have too many charms :P)

Anyway I wanted to see the full value of each tier for the sake of real comparison (we will take the normal shop prices):

25 milestone = 125 + 45 = 170 coupons (total 170)

50 milestone = 250 + ??20 = 270 coupons (total 440)

150 milestone = 375 + 135 = 510 coupons (total 950)

250 milestone = 625 + ??405 = 1030 coupons (total 1980)

500 milestone = 1250 + ??600 = 1850 coupons (total 3830)

1000 milestone = 1000 + 1800 = 2800 coupons (total 6630)

stamina potion doesn't have a real price so let's guess around 20, magatama is a random pack so could be 108 or 135 depending on which one you get, and lvl 5 rainbow magatama costs 600 coupons in konoha gift shop

based on this full value comparison:

250 milestone 2 times = 3960 coupons value vs 3830 coupons value for 1x 500 milestone

500 milestone 2 times = 7660 coupons value vs 6630 coupons value for 1x 1000 milestone