ebay posted a postage price that was lower than what shipping cost me-the buyer won;t pay the remainder

The mistake is yours, not eBay's and it has nothing to do with the buyer.

The buyer needs to pay only what the listing shows.

You cannot ask the buyer for extra money.

If you do not ship the item for the advertised rate, be prepared for negative FB, low DSRs and being reported to eBay as a non-performing seller.

I'm sorry, but the buyer is only responsible for the shipping price shown in the listing.  You can't invoice for additional funds after the sale.

It is your responsibility to figure out shipping methods and rates before you even list the item, and now you're responsible for the remaining balance.  ebay doesn't set the postage price, you do.  If you use calculated shipping, the price is based on the particulars you enter into the system.

If you refuse to ship, you risk negative feedback, a non-performing seller strike on your account, and low DSRs.  I'd say take this as a lesson, and ship the item to the buyer.

“Don't spend time beating on a wall, hoping to transform it into a door.” - Coco Chanel

Ebay does not estimate shipping.  They calculate based on the information YOU put in.  You put in weight, measurements & shipping options.  Then buyers can get the shipping costs based on their zip codes or locations.

You cannot ask the buyer for more money, Ebay states that you cannot change the terms of the listing after the sale, which means you cannot ask for more money.  The only time you can ask a buyer to wait for the invoice & pay more shipping is if the item is shipping international & you did not have the shipping set up in the listing

If you refuse to ship because of this, that is your option, but be prepared for the fall-out, which means the buyer can report the item not received (dispute against you).  They can report you as a non-performing seller (black mark with Ebay status).  They can leave you neg. FB & low DSR's (no help in future selling).

Many sellers make mistakes, but they deal with it & learn from it.  It is extremely dangerous to guess at a weight, as you now know. 

Wow did same thing. Yeah they have there rules bu, it's funny you have no control of your own merchandise. If a buyer doesn't pay. There's no recourse for the seller. Can't leave them negative feedback or anything. You are bound by rules as a seller con buyer when it's convienent for them. I personally get it to trouble when. I have to ship Midwest or West coast it's always more ( in southeastern state) it's a double edge blade because if you charge to much For shipping you get fewer bids. if you use chipping calculator it over inflates and under I inflates shipping. Leaving you stuck with more shipping or fewer bids as people think you are overcharging on shipping. I've shipped a lot. Still get emails why shipping so expensive. I once shipped a plastic container small lightweight. Started at .99 shipping 13.00. All kinds emails pored in my shipping too high. In the end got 1 bid for .99. Shipped to Cali price 14.00  I successfully paid somebody buy something from me never again. I started putting in auction that Midwest and west states I would charge an extra 5-10. still got arguments when having to use it. I don't cheat anybody on shipping. I ship usually by USPS postage clearly marked on package. Never get a thanks  when shipping is more and I pay or when Ive refunded some bodies shipping for paying too much. Nobody cares. If i as a buyer don't want to pay shipping charge wether I think it's too high or not I want bid. If somebody emails me and states that shipping to me is going to be more I would have no problem paying . Especially I know they are further away and item is heavy its common se