With the coming SG scratch, and possibly many other things to spend SG on, there isn't a whole lot of ways to obtain SG in the first place. SG is 20 x 1.00 USD, so a 80 SG scratch would effectively cost 4.00 USD to purchase. Given SG for the most part is a free currency, and may have applications in the future to obtaining it for free, it feels like that it is still a very expensive process of obtaining the currency.
I suggest a method of obtianing it automatically as rewards for spending the AC currency. If you want to buy scratch cards, inventory expansions etc, a sum of that AC spent would reward 5% ~ 10% into SG. That way it gives more incentive to buy more AC, and spend more AC & SG on cash shop purchases.