Letters prepaid, unpaid, short paid

All letters or other articles from any country which have been prepaid according to any arrangement between Belize and any such country, shall pass through the post offices of Belize free from any further or other postage or charge thereon, and on letters or other articles to or from any such country, whether wholly unpaid or insufficiently paid, paid. 16 there shall be charged such addition to the ordinary rate of postage as may have been agreed upon as aforesaid.