Given that you are a new seller, and that overseas selling is somewhat riskier than North American, and that this is a relatively high value sale, repeat to yourself: some sales are not worth having.
Take a deep breath. This may or may not work out. It's a learning experience.
Yes. Refund the short payment and re-invoice the buyer with the shipping fee included. If he pays, ship using Air Mail service only. Given the value , including the shipping fee, you should also be paying for Confirmation of Delivery. That would be ExpressPost International*, which is expensive, but your buyer is paying for it not you.
Air Mail means it will arrive relatively quickly. You won't have an unhappy, honest customer getting a refund because the item does not arrive within 45 days.
Delivery Confirmation means that you can prove that a dishonest complaining customer did in fact receive the item and Paypal will not refund him.
What if he doesn't pay the new invoice?
Well, everything being considered, this is a risky first sale. You may be better off without it. You can't open an Unpaid Item Dispute since he did in fact pay.
Hmmm- did you originally have a shipping fee for Russia? Did you have blocks against any countries, like Russia? It may be possible to end the transaction and get your selling fees back by reporting the bidder as an Unwanted Buyer.
BTW, did you know that although you are required to ship within 7 days of payment (complete payment), PP will hold that payment in escrow for 21 days against your good performance. They do pay out, but the delay drives some new sellers nuts.
*The new Tracked Parcel, if it is available for Russian delivery, may be somewhat cheaper. My books are cheap so it isn't something I have personal experience with.
