How do I determine the "worst case" shipping on Ebay?

The best way to go about it is to box up your item and then weigh it. You can use eBays shipping calculator that will tell each person who looks at your item how much shipping is going to be to their location. If you do not have an accurate scale you can take the package to the post office and they will weigh it for you.

Guessing is a sure way to make someone very unhappy. Usually yourself. If you underestimate the cost then you eat it. If you overestimate the cost you have to do a partial refund or the customer may become angry and leave you negative feedback.

If you have no plans to ship outside the US, (or where ever you are), you need to say so in your listing and change your settings on eBay.

Depending on your item, eBay does have a shipping calculator which works on size or package and weight. Otherwise you can put in the listing, please contact me for a shipping quote before bidding (that's more for if you are going to offer international shipping and of course then the cost depends on the destination). Most buyers with experience know they usually have to wait for an invoice ie after winning the item the seller has gone up to the post office to get a shipping cost and updated the invoice for that amount.

If you live in Maine, figure out the postage to Hawaii or Alaska.

Well, put it like this... Is it in your same city and not to heavy? OK, look it up on usps.com per product and walaa, problem solved!!