You forgot paysafe. Its easy and safe.As other have said already, there are ways to do it, but you wont get help on this forum telling you how, and if you so happen to have an issue with the game after it's released, no one here will help you fix it unless you have a validated copy of HL. You can use PayPal to buy games off steam, you can even go out and buy a visa gift card with $10 on it (or whatever your countries form of currency is) and use that to purchase through steam.
Not to be rude amigo, But the game only costs like 10 dollars.Title. Sorry if this doesn't belong here.