[Bungleau]

I'd head down to an office supply store (Office Depot, Office Max, Staples, etc.) and check their prices on packaging. Then divide it out to find your per-package cost. Things like dunnage (the internal packing materials) are harder to estimate... fortunately, a roll of bubble wrap is fairly cheap.

For the sizes you appear to be talking, I'd add a buck over the cost of the box and you should be set. Either that, or round the box cost up to the next dollar.

And yes, you can set minimum bids. There is a fee for that, so be sure to factor that in.

Hmmm... seems to me I put together a spreadsheet when I was listing my first couple of items that helped me figure out the Ebay fees. Drop me a PM with your email, and I'll dig it up and send it over.