I have said it before and will say it again you do not buy kits, kits are for occasional shipping, if you plan on doing this seriously you buy supply for the season and as needed. (I buy 20 boxes at the time in general)
After that you have choices, charge the exact shipping which usually will be the price of the label, seler usually do not charge a box fee, that's part of doing business.
Of you can charge a flat rate which means you provide a low shipping cost for your customer and absorb the rest, that's what the majority of sellers do (again cost of doing business), I for example charge $50 no matter where I ship or how big the box is, and in reality shipping usually cost me $60 to $100.
Shipping overnight is expensive and obviously some animals will be worth shipping for you and the buyer while other will not (whether it's for you or the buyer)
Finally Fedex use dimensional weight so the size of the box is what matter when shipping reptiles, not the weight.