Why not politely say,," Excuse me Mame, can you ring me up real quick before you help this gentleman?"

Based on your description she seemed very nice and would have help you get on your way. Sure she could have asked you if you were ready to go, but who knows what was going through her head, maybe she assumed you were going to browse the store some more. I do that just about every time I go into a hobby shop,,many others do to

But no, you sat there like a dear in the headlights and watched the whole thing happen then come here to Vent???.
You did nothing and assumed she knew you were ready to go. If you felt you were disrespected, you should have said something and walked out.
I'll never buy something from a vendor that is disrespectful.

People aren't mind readers, Be assertive Man!