purchashing downloadable software and tax exempt

Greetings,
I am a purchasing agent for a Government agency and would like to make a purchase for another employee. How do I go about getting the software download to the employee if I am the one making the purchase? Also, we are tax exempt and should not be charged tax. Do you need our tax exempt letter prior to the purchase??
A reply asap would be greatly appreciated.
Regards,
Elaine Peterson
Naval Research Laboratory
202-767-5369