Combined shipping - adding tracking number to items paid separately

[skwishmi]

Hello eBayers!  

Is there a way to add the same tracking number to more than one sold item?  Maybe I'm missing some option at the point of label purchase?  It keeps happening - people pay separately, but I ship everything in one box, and then can't add the tracking number to the items not used to print the postage.  There has to be a way, and I'm just missing it... right-?  Help, please?

-skwishmi