I'm sorry but that's not possible. All upgrades must be paid for at the time of purchase by the blog's owner and original Admin who registered the blog in question. No purchase orders can be used. After that purchase is made the blog's owner can go to > Dashboard > Store > Billing History is where they can print out their own receipt.
Below are the only accepted means of payment.
WordPress.com uses PayPal as the payment processor, and as such, can only accept payment from countries they support: https://www.paypal.com/worldwide/
If for whatever reason, they have chosen to block payment from your country, you will not be able to purchase an upgrade here unless you have a source of payment outside of your country.
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 http://en.support.wordpress.com/?s=bitcoin
Note: You can buy a prepaid credit card from an approved country or pay in Bitcoin.