When shipping items withing the Jambi Province, and a few other locations on Sumatra, two good services are:

TOP Travel & Express, and Ratu Intan Permata.

You can usually send a single #10 envelope on the same day for the equivalent to $1.10 US   (Rp. 10,000).  Larger packages can also be sent via this service.

Large bulky items (like motorcycles) can also be sent relatively inexpensively.