You don't really have to do too much. If you know the dimensions and weight of your package, eBay will give you a range of cost depending on buyer location. For example, it may list that the package will cost between $14-50 to ship. With that, I'd estimate an average cost of $20-25 where the higher ones are likely significantly further away. If getting package weight is a hang-up, I use a kitchen scale for smaller packages or a bathroom scale for larger ones. If a package may be larger but not weigh more than 10 pounds, I'll use the kitchen scale with an item to prop it up so I can still read the readout.
One thing that I kept running into a problem with -- and I think some sellers missed this based upon their shipping costs -- is that eBay doesn't always handle parted out items very well. A great example of this is when I was selling my Vive in pieces. The Vive was never really sold in a piecemeal fashion outside of replacement parts. (You could buy official lighthouses, but that's it.) So, when I'd try to sell something like the headset itself, eBay's suggested package size and weight was based upon the size and weight of people selling the entire Vive kit. I mean... I don't think the controllers weigh 18 pounds.
As a side note, packaging is also an important aspect. If you can, try to get your packaging to be as close to the size of the item being shipped. This can be kind of hard depending on the item. I'll be selling my PlayStation VR soon, and it hasn't been easy finding a box that nicely matches the tall, rectangular shape of the product box. Oh, and one thing I do is use grocery bags as packaging support. It sounds weird, but they're lightweight and allow for very little to no movement if packaged properly.