The "oh hi" died in her throat as soon as Mary Lou started speaking. Mellie felt her mouth open to form a little 'o' and she turned to Amanda on her left to see if she was reading the situation right. This was a bit much, wasn't it? She didn't go out of her way to make Mary Lou feel dumb, did she? A voice in her head said that there was probably very little that made her feel dumb, since the fellow Wampus was so much smarter and better at things that mattered, unlike her, who spent most of her time reading fashion magazines and relationship advice. She pushed that voice away quickly. She didn't need insecurity right now, she needed righteous anger.
That same anger also meant she could feel her face starting to burn, and she didn't need a mirror to tell her that it was probably getting red and blotchy like how it always did when she got upset, and Mellie hated being upset. In fact, she hated it almost as much as looking ugly, but right now she's pretty sure she's a combination of both. "I know when the bake sale ends, I was the one who even gave you that dumb proposal to look over," she rolled her eyes and picked at her polished nails testily. She knew Mary Lou tended to get intense with her proper "procedures" and paperwork, but sheeeeesh.
"Of course I know I signed up for the whole shift, but I think you should also know that this entire shift wasn't going to make much profit anyway." Mellie never played around when it comes to things like customer flow and strategic positioning for maximum sales; she might not be smart in terms of theory or doing math or whatever, but this was practical knowledge stuff, and she definitely had the upper hand over Mary Lou when it came to stuff like this. "Honestly, this is just a marketing ploy to make the SGA look good, and make some of us look like we're doing anything at all." The 'I'm doing you a favour really, no need to thank me' wasn't said out loud, but Mary Lou should be able to infer the implied meaning.