So, you have two medium-sized envelopes to send through the pouch to Baltimore (for free) and then from Baltimore to various parts of the country (add postage stamps purchased from the States before leaving to Africa)
1. Hold the two medium envelopes in your hands
2. Guesstimate their weight. The average weight of a laboratory brown mouse is 25 g (from your experience working at the NIH). The two letters weigh about the same of one mouse. The two letters are about 25 g combined.
3. Go to a site converter website. Type in 25 g to get the pound value (25 grams is about 0.055 pounds). So it's about 0.025 per letter
4. Go to the US Postal Office website. Type 0.025 in their "Postage Rate Calculator". Determine that 1.2 stamps is approximately the correct postage.
Ask Congolese Secretary to confirm this. As she is not from the United States, she is unable to answer but suggests you put two stamps on each letter as they weight just a bit more than an average letter. D'oh.