Simplest way to add a second shipping method at buyer cost if originally offered as free

[friskya]

I've got an item (#254922324807, since I already know someone is going to ask) that qualifies for Media Mail shipping (yes, I also know that I won't be able to purchase that from eBay) and I've got it listed for free shipping. 

I want the eventual buyer (assuming there is one! LOL!)  to be able to choose to pay for upgraded shipping to First Class. I know I can add the additional service, but I don't see that there is a way to specify that it's to be buyer paid. In short, either free shipping for media mail, or buyer pays for first class.

What am I missing? Or is it not an option except by removing the immediate payment and telling them to await my invoice?

[nobody*s_perfect]

You don't need to specify that the buyer will pay.  Buyers are always expected to pay for shipping unless the seller designates it as Free.  When you revise the listing to add an additional shipping option, you will have the option to enter a cost that the buyer will pay for it, or to set up calculated shipping.  

I'd set a flat price based on the difference between the cost of MM ($2.89) and the cost to ship to Zone 8. So for a package up to 12 ounces that would be $2 since the cost of shipping the item  to Zone 8 would be $4.68.  If the package weight is up into the highest price bracket (12+ to 15.99 ounces, which costs $5.98 to Zone8) set the increment at $3.