for a 1/3 sheet cake, iced in buttercream, one layer torted and filled with strawberry filling, with a fondant onesie, baby shoes and scroll plaque

i have never worked with fondant before so have no clue what to charge

What's a third sheet cake? I do mine by the serving so I would multiply the max serves by the price per and add on for the fondant work. I know different people use varying sizes for sheet cakes.

11x15x2

Probably $40-$45. My base price for that size is $35, plus fondant and extra work for decorations.

I would say at least $45.