Are you trying to implement your own tracking system or just provide a way for users to enter information on your page and be shown the results on USPS.com?
If it's the latter, you can do this:
<form action="http://trkcnfrm1.smi.usps.com/netdata-cgi/db2www/cbd_243.d2w/output" method="POST" name="getTrackNum">
<INPUT TYPE=HIDDEN NAME=CAMEFROM VALUE=OK>
<input type="text" id="label_number" maxlength="34" size="22" name="strOrigTrackNum" />
<input type="submit" value="Find It" />
</form>
Otherwise, I'm not sure they'd be willing to share their code and resources. But I guess it wouldn't hurt to try.