You can generally order via purchase order, unless the publisher of the product you wish to purchase does not offer this option.
If you have a reseller/key account with the publisher, the "Purchase Order" option will be available in your Reseller/Key Account Panel.
Customers with or without reseller/key account can generally choose to submit a purchase order request online. If this option is available, “Purchase Order” can be selected in the payment options section of the online order form. Most publishers offer this option to corporate customers only, so once you select that option, you will normally be required to provide your official order on company letterhead with a valid signature or company stamp in pdf format. Upon submission that request will go into review by the publisher and you will be notified once the request is approved.
The approval email will include an order invoice with payment instructions as well as delivery information via email.
Note: If you do not see the "Purchase Order" option on the order pages, then unfortunately the publishers of the product you wish to purchase does not offer this option or it cannot be offered for your country-currency combination.