The only other way to buy is using PayPal at the moment.
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We use PayPal for our credit card processing, and there is an issue with them and Russian Federation credit cards.
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We had hoped that this would be resolved, but it has been like this for much longer than we'd hoped. We are a bootstrapped startup and have very limited development resources, so it won't be until either January or February 2018 until this is resolved by us, unless PayPal fixes it first.
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Sorry about this.