Puzzle of the Week #1 - Strange Stamps

The islanders of Fictitia have a rather eccentric postal system. Postage for an item can be anything from 1 dinar to 12 dinari, and you must use exact postage.

Frustratingly, there is only space on the envelopes in Fictitia to attach a maximum of two stamps.

What is more, they only have four different denominations of stamps, can you work out what they are?