Also, it takes the purchase credits screen a few seconds to fully load. For example, there are times when a promo is automatically set on the purchase, but it you click too fast and complete the purchase before it loads completely, the code won't be applied. This could be the same in your case. You click $25 before it is fully loaded, but once it does load, it resets to the default setting of $100. This happened to me before, but when I wanted to buy more than $100 worth. I was confused why I didn't have the right number of credits, but then I realized it was processed for $100 instead of the amount I wanted.
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