How do I take only a portion of the freight at this time? What if I don't want any of this freight on the first receiving session?

When you entered the cost packet on the PO you need to check the flag "Allocate per Receiving". This will allow you to distribute the freight over multiple receiving sessions.
 
If you do not want any of this cost packet on the first receiving session and only on the remaining sessions, then when you are in your 1st receiving session, you will want to open that cost packet and zero out the cost. This will push the amount back to be distributed over the remaining receiving sessions.