I'm not sure if this is allowed, so sorry if it isn't. I'm from SWGOH, where there are a lot of top-up services that essentially sell diamonds cheaper than in-game.

Does something like this exist in this game?

Yes, third-party top-up services always appear in these games, and many now do this through LootBar, where you can top up last war at a better price than directly donating in-game. It's essentially the same type of service you're talking about. It's a top-up store that offers discounts based on regional prices, promos, or other pack purchase schemes.