You should add a 'skypeout' like option to this where it works internationally.
People in the country of destination would print out the items and mail them for the cost of a local stamp and get some compensation for their efforts.
That way you can offer international 'real' snailmail for a tiny bit over the price of a local stamp.
You could do that domestically also. This could be a way to scale it. Allow people to sign up for "mailer" accounts, where they can print out the letters and mail them for a small compensation.
People in the country of destination would print out the items and mail them for the cost of a local stamp and get some compensation for their efforts.
That way you can offer international 'real' snailmail for a tiny bit over the price of a local stamp.