Make another currency, like IGG coins - equivalent to the amount payable. The packs or items only payable in cash can now be purchased with IGG coins.
For example: 1400 gems are currently payable only with $10. I will buy 10 IGG coins with $10. With IGG coins, 1400gems can also be purchased using 10 IGG coins. Other packs, items, and events that uses real cash amount can now be also purchased with IGG coins.
I do not know why IGG do not implement this yet. This is the most capitalist idea ever. A player may spend only at a future certain time. However, with the  IGG coins, the money was already paid to IGG in advance before the time of the actual spending or use in-game. This may also have the tendency for players to spend more money.
I was talking about NEW GEMS.
EXAMPLE: BEFORE, GEMS was the PRIMARY tender to buy everything. NOW, there are already other packs that cannot be bought by gems. With IGG COINS, we use real cash to buy it, then spend IGG COINS to buy any packs. There is no need to always go to google payments.
I can already see potential problems with this. IGG is a gaming company not a banking one so they would have to invest in a whole new department just to get started in this industry. International banks cost a lot to run so unless they have been saving a decently large sum of money up they cant even think about starting this. IGG its self has tons of games they run, so would they only implement this bank into their most profitable games? How would they regulate which games get this ¨perk¨? Its not a bad idea tbh but its just not realistic as of right know.
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As Shadygamersan stated, IGG is NOT a bank. You do realize that when funds are deposited into banking institutions, they generally charge monthly maintenance fees, which would be the only incentive IGG would have to even consider such a venture. Are you prepared to pay these types of fees? I am sure many will not.
The reality is that any method you would use to purchase these proposed IGG coins could be used to purchase any items you would like in game without any additional surcharges other than the applicable taxes.
Finally depositing funds ingame would not be an incentive for anyone to spend more money, and I don't see any refunds being issued for any unused coins.
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So basically you want another "gems" equivalent but this time something which would allow those who have it, buying everything from game whenever they want, right?
In my opinion it would require a lot of work to implement it into game. Game code would have to be changed from scraps, and this could possibly make more bugs and errors. And this is place where i see biggest problem and possible devs unwillingness point. Why to change something which is working just to "improve it".
What it would give? Option to make purchase earlier but use when you want. But why not to make purchase when you need it, like now? Only difference would be that you are making purchase earlier.... And i think this is reason not good enough for it to happen, considering whole work devs will have to make to implement it.
Personality i would prefer if that time would be used on adjusting rewards instead.