They know the thicket of rules and petty fiefdoms, what rules apply and which don't, what to pay and to whom... Regular post just acts as if everything works by the book - and that doesn't fly. Use word-of-mouth to find the good couriers, trawl through your local community of people from the destination country - it is a very common service, so you'll soon find a good provider. Test it with a couple of low-stakes deliveries and you'll have a solid channel.
Meet your guy in a metro station, or find the shop in Barbès that smells like a marketplace across the Mediterranean, hand over your package with the recipient's name, destination city (Addresses ? Where we're going we don't need addresses !), your phone number and the recipient's phone number scrawled on it with a felt-tip marker (make sure they are Whatsapp numbers), pay in cash, don't get a receipt (lol) - and there you go !
Operating in Africa will soon tire you if you attempt to force European ways. Going with the flow (with appropriate caution - a nose for issues, borne from experience, is invaluable) works and makes the experience enjoyable !