Many sales transactions involve the simultaneous transfer of goods and payment - as in a shop. Other sales take place at a distance, but involve parties who know and trust each other - such as purchases from Amazon. In other instances, the buyer can rely on promises/guarantees given by the intermediary - eBay - or there are laws protecting a purchaser at a distance.
It's a different story when the parties are in different countries. The seller may not know the buyer well and vice versa.
Hence, the problem: the buyer doesn't want to pay before it receives the goods and the seller doesn't want to part with the goods before payment.
Imagine that you work for a company that wants to enter into an agreement to sell goods to a buyer in another country - but your company wants payment in advance and the buyer wants receipt of goods before payment.
What would you advise your boss to do? How could this problem be solved. Use your imagination, but try to read up around this issue and if possible provide citation to authority to back up your suggestion.