[The Quick Answer: try making your purchase again, but use the PayPal option, not our credit card form.]
We're sorry to hear you're having trouble making a purchase and you're seeing the "risk model" error message. This is rare, but it has been reported to us before, and with the advice in this article, you should hopefully be able to get this issue resolved.
We use PayPal Website Payments Pro to process our credit card form, and they may be blocking your payment to help protect you from fraud.
It is also possible that your financial institution, like your bank or your credit card provider, has blocked the payment in order to protect you from fraud.
Please note that on rare occasions, even though a transaction has been blocked by your financial institution, it may temporarily appear that your account has been debited for the purchase. If this has happened to you, you should receive the funds back in your account soon. If you do not get the funds back soon, please contact your financial institution and let them know what has happened.
Just to be clear, when you see the "risk model" message, it means someone else has blocked the payment, not Leanpub. It also means Leanpub has received no payment from you.
As a next step, we recommend you try checking out using the PayPal option we provide when you try to make a purchase, instead of our credit card form. This process is separate from the process PayPal uses to process transactions from our credit card form, and sometimes payments work using PayPal directly, even if they don't work when you use our credit card form.
You could also try paying again later, or with another card.
Unfortunately, in these cases, we are not provided with any more specific information that could help us determine the exact cause of this issue.
If you do try the PayPal option and it still doesn't work, what is probably happening is that your financial institution has blocked the payment in order to protect you from potential fraud. In that case, the best thing to do would be to contact your financial institution, and confirm that your purchases from us are legitimate purchases by you, and ask them to "whitelist" purchases from Leanpub made by you.
If you do need to contact your financial institution, here is some business information about that might help:
Finally, the problem might be that you are trying to make a purchase from a country PayPal does not support. Here is a list of countries supported by PayPal: https://www.paypal.com/us/webapps/mpp/country-worldwide.
If you are unable to purchase a book because PayPal does not support purchases in your country, we suggest that you go to the web page for the book you are trying to buy, and you scroll down until you see the "Email the Author(s)" button. You can use that feature to explain the situation to the author, and ask for a free coupon.