The US$250 is dependent on the site that the buyer was logged into when he bought your coins. If he was logged into ebay.com.au, you just need proof of postage and registered post international would be OK (though it only includes $100 insurance). If he was logged into ebay.com, you need the signature confirmation.

Have you already sent an invoice, and do you have a copy of it in your mailbox? If so, have a close look -

There should be a line at near the very bottom  like

eBay sent this email to you at sellerebayemail about your account registered on www.ebayXX

Now, if the buyer bought on ebay.com.au, the ebayXX will be ebay.com.au, if they bought via .com it will be ebay.com, via ebay russia it will be ebay.ru etc. (Actually, if they bought via ebay.ru it will be really obvious, as there'll be loads of Russian text in the invoice. However, if your post-to locations don't include russia, he won't have bought it via the Russian site).

Given the buyer's location and the initial US address, I would say it's most likely that he bought via ebay US, and you need signature confirmation, which will be expensive. (eg express courier international with insurance is about $86 for 750g). 

I've posted cheapish items to Russia (less than $100) - the buyers all asked me to post registered/signed for and they did arrive - I got the impression that unregistered mail was less reliable.

I would be phoning AP on 13 13 18 to double check whether ECI will actually get an online record of signature if sent to Russia. Also, ask them whether registered post international is trackable at the Russian end, as (if you trust the buyer) it might be cheaper to post in two separate parcels (with two separate payments both under $250).

Have a hard  look at the buyer's feedback. In particular, how long has he been registered and buying goods? 

Remember, he needs to enter the St Petersberg address in checkout when he pays, you only post to the address given in the paypal payment.

As another aside, are you coins being bought by collectors, or by speculators?  If the latter then you need to be extra careful about fulfilling all the ebay/paypal requirements for seller protection.

Good luck