Registered Post International is a flat rate of $5.40 more than regular International Air Mail.
To offer Registered Post International, you can create a new freight rule in ezimerchant and call it 'Registered Post International'.
Choose International Air Mail from the list and turn 'Calculation Rules' on.
Select 'Everywhere' as the region.
Then the code for postage is going to be FreightCost + 5.40