Namibia

Sent from Windhoek: the capital of the south-west African nation of Namibia. The souvenir sheet stamp shows the shape of the country in the colours of their flag. That little bit at the north-east corner that juts out between Zambia and Botswana, is known as the Caprivi Strip. It was acquired by the Germans when Namibia was known as German South West Africa. It gave them access to the Zambezi River. From there, technically, you could reach the Indian Ocean but you'd have to somehow manage your way around Victoria Falls. 

The butterfly on the 1993 stamp in the lower right is a Green-veined Emperor. And the minerals shown on the definitive from 1991 are wulfenite crystals.