She may incorporate the shipping charges she pays (and she definitely may pay depending on what she's ordering and from where) into what she charges you...but to ship it to you when you stated that you'd be picking it up is wrong.
Maybe most people prefer that she ship things to them and she just forgot what you'd said about picking it up.
I'd chalk it up to miscommunication and pay the $8 and move on...from here on, make sure you specify clearly that you'll be picking something up so that she can put a note in her records.