If I shipped more often I'd buy the supplies in bulk and only charge shipping. But as it is, I only ship a few out a year so it's just easier for me to buy kits as I need them. Unfortunately, that means I have to charge a bit more for shipping.
I charge pretty much the shipping cost. I do round up or down to the nearest dollar, and I eat the cost of the boxes. I buy them in bulk about 1 x a year. So i figure it as a business expense.
If shipping any sort of volume you would be better off to order your supplies in bulk from superior enterprises. However if you are just doing a handful single ‘kits’ might be better suited for you. What you charge is totally up to you. It’s hard to eat charge $35 for shipping when the actual cost is $43 + cost, especially if it’s on a cheap animal. I always build a few extra bucks into the posted animal price to pay for shipping goods then charge actual shipping. But it all depends on how you run your biz.
I just have the customer pay for shipping and purchase the supplies for shipping myself aswell. I think it's crazy to try and make money off of a box and a deli cup aswell, lol.