Official comment
Postcards can be sent to any US-based address, one at a time! They can be sent via a Step or Date Sequence, so you can send a "Happy Birthday" postcard to a single contact automatically. They use the same merge fields as an email message, so you can have a one-off, personalized post card sent to a contact with their full name on it.
For billing purposes, you can buy print credits in $10 increments. The more print credits you buy the lower your per-postcard price is, but keep in mind the credits are non-refundable. When a postcard is sent, the price of the postcard is deducted from your balance. If you reach $0, ONTRAPORT will re-load your print credit account with another $10 and bill your credit card on file so you don't run out of print credits and miss sending out those cards. Here is the account screen where you can find the Print Credits link:
When you click that link the following table pops up:
