I don't know if this has been done already, but what I think would be cool to add on to special offers is reserve stamina.
(the 20% x 25 stam per fsp upgrade is what I think would be fair.)
$5 - 25 fsp + 5fsp(bonus) + 125 reserve stamina
$10 - 52fsp + 10fsp(bonus) + 250 reserve stamina
$60 - 400fsp + 80fsp(bonus) + 2000 reserve stamina
Having the reserve stamina instead of epic potions/chests is becauase then you can use this for events. Lets say, we have a global event, and you just hunted. Well, because you spent $120 on the last special offer, you have 4000 reserve stamina. Its not made to be used for every special offer either. Every third one, every other one. Something of that nature.