We're sorry, we don't process purchase orders. At Leanpub, we only accept payment via our credit card payment form, or through PayPal.
Please note that after you make a purchase, we do have a feature that lets you add company information to the purchase record, and then lets you send yourself a new receipt via email, that includes your company information. Here are the instructions for doing that:
http://help.leanpub.com/reader-help/i-need-to-add-company-information-to-my-receipt-how-do-i-do-that
Finally, if you are having a problem making a credit card purchase on our site, one thing you can try is choosing the PayPal option and then entering your credit card on the PayPal site. We are a Canadian merchant, and we use PayPal Website Payments Pro for credit card processing. However, sometimes a credit card which does not work on our site ends up working when used directly at PayPal.