You have to learn when to stop buying packs and walk away.
The store is not a store at all. It's a casino/torture device. It's a psycological contruct. The following is how I deal with it.
I get above average drop rate or I walk away. It's that simple. If the store ****s on me, if it fails to reward me, I fail to reward it. I walk away.
The concensus observed UR drop rate on PSPs is 9 to 11 percent. I am worth better than that. The proof is that I can just walk away and nothing can stop me. I aim for at least a 25% drop rate for URs and gear(just as rare as URs if not more so) in PSPs. If I don't get what I want, then the casino failed so I stop rewarding it with my credits.
25% drop rate is one in 4 packs. So, I buy in sets of 4. I stop buying after a bad set. 1 UR/Gear in 8 PSPs is 12.5% which is ****. It's become very easy to hold onto my credits until the store goes through a generosity streak for me. I've had great luck and little bad luck since I started doing this. I've had drop rates as high as 50% and in hind sight could have had higher if I had not bought the last few packs of those runs. But, my strategy still paid off. So, I remain on course.
I aim to save 1.2 mil credits because I have never seen a luck streak go beyond that. I have saved up to 1,2, 3, 4, 6, and 10 mil credits in the past. So I have had some opportunity to watch the store's generosity or lack of.
Any credits beyond 1.2 mil, are my testing credits, in sets of 4 PSPs. Even if I'm sitting on a mountain of credits, I don't spend until I play enough matches to be able to buy 4 packs. So, if those 4 packs are garbage, my credits stash has not been diminished relative to when I decided to buy. I have to "work" for the credits before I spend them. They are "hard" earned. So, not to be spent lightly. Should a long generosity streak occur for me, I'm ready to reap it. So, it's worth protecting my credit stash.
The store algotrithm is based on a RNG, no doubt. But, it has been observed many times to have random generosity streaks.