Snail mail your customer the account, password, and SteamID via USPS Priority Mail, MAKE SURE TO GET A TRACKING #.
If someone performs a chargeback, forward paypal the tracking #, federal proof that the goods reached the destination. Paypal will side with you, and return your money.
The reason Paypal usually sides with the person who performed the chargeback, is because there is no physical proof that something was sent, it is all electronic goods.
Especially nice if you're selling multiple accounts, with totals over $200.00. You can take someone to small claims court, or arrested for fraud.