At Leanpub, we only accept credit cards and PayPal.
We do not accept purchase orders, sorry.
You can get an invoice which shows your VAT details after the purchase is made. You can even add custom information to the invoice, which may be helpful for your accounting department.
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.