I've shipped boxes for 25$ plus the 90$ for the fish (pair of adult albino BN plecos). I've also shipped small boxes of just plants for 8$ or so. All depends on how much the box weight, how big it is and how far it needs to go.
Then there's the cost of the boxes, bags, O2 tank, Prime, rubber bands, sytrofoam/insulation, tape.... I'm not entirely sure I'm even coming out ahead at this point. At least my boxes are free of charge.