I'm sorry but that's not possible. All upgrades must be paid for at the time of purchase. No purchase orders can be used. After you make your purchase her > Dashboard > Store > Billing History is where you can print out your own receipt.
You must be logged in under the exact same username account that registered the blog who is Admin of the blog to purchase upgrades for the blog.
Dashboard > Store > My Upgrades
Below are the only means of payment available.
WordPress.com uses PayPal as the payment processor, and as such, can only accept payment from countries they support: https://www.paypal.com/worldwide/
In addition to major credit cards Mastercard, VISA, American Express, Discover, and PayPal https://www.paypal.com/worldwide/ WordPress.com can accept echecks and some debit cards.
The debit card must have a VISA or Mastercard symbol on it, and should work just like a credit card when selecting the VISA or Mastercard option on the payment form, even though it's a debit card.
More about echecks here:
WordPress.com also accept Bitcoin for payment of certain upgrades ( bitcoin cannot be used for domain and premium theme purchases)