It will only 'go against' you if he files an Unpaid Item case. Although that would be hardly fair, since his settings are causing the problem. To ensure this does not happen, ask the seller to send you a Request for Cancellation. The following link will tell you about it, and you can forward it on to the seller if need be:
http://pages.ebay.com/help/sell/cancel-transaction-process.html#cancel
When the Request appears in your ebay messages, be sure to click to respond and click to agree. The seller gets his fees back and it brings an end to the transaction.
That's my suggestion, anyway. Otherwise, the seller can mark it as 'paid' but you will be hoping for the next month that he doesn't forget and file the claim anyway.
As for sending the invoice to your regular email, - he doesn't need your paypal account, only your email address, which he ought to have already if you have clicked 'buy' and the item is in your Purchase History, - but if he sends you an invoice with items and a total, all that, - that is just like buying anywhere. What you do not want to do ever is use the 'send money' option in your paypal account because that is not buying, but giving. When you buy, you have buyer protection, not donation protection.
In future, though, it would be wise to ask a seller in advance if he is willing to ship to a PO box. Many will not, and you may continue to run into this situation as long as that remains your address.
