I am asking the support team to see how it goes. I might have to change the game to a paid game to resolve this issue =.=
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Well, I have the answer.
In general, if you pay $10, you can freely download all files that cost less than or equal to $10, regardless of the version.
However, this function does not count the total amount you pay. If you pay twice with the amount of $2 and $8, you can only download files with a maximum price of $8, not the total number 2+8 = $10 that you want.
So, there will be a problem if I increase the price later, for example to $12. Then clients who have paid $10 will have to pay exactly $12 more to download this file, they cannot pay just $2 more to download it.
I promise not to increase the price, just keep it the same or reduce it :D