Is it permissible in Islam to bribe someone out of compulsion?

I am an eBay seller and I need to ship goods to customers around the world. But custom officers in my country are corrupted and they ask for bribes from me on each shipment. Now I am using a courier service. The person who manage the customs told me to write lower values on the item invoice. I am doing so from then but I think it is not right. Is it permissible in Islam to bribe the person if he otherwise won't do his job legally even though he should? Can I under value the items because the courier officer asks me to and wouldn't let me do my business otherwise? In both the cases I'm compelled to bribe them for something that I have legal right to do. What is Islamic ruling in this regard?