Same thing. Send the text. Waaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaait for it to return with instructions. They're very slow. But it's very convenient.
I would even suggest you talk to the post office that you'd like to be receiving from.
The notifications come from a post office guy itself not some number.
Since it's basically a manual text saying kujia package, just try putting your number as your address and mail yourself a letter and see if it will work. 0721345678-00100 for example.
Sometimes I wonder why exactly I registered for it when in reality notifications tu ni kutoka ule mse wangu wa posta.