I'm a buyer, not a seller, so here's my take on it.

First off, no matter if the ad is in the last few minutes or not, you can only take the ad at face value and if there is no time for questions and answers, bid on the assumption that the prices listed there are what you will pay. Sure, some sellers do combine, but it can't be assumed.

Second, no matter how cheap the thing is you are buying, that isn't really relevant to the postage cost. A coin might be a rare one worth a small fortune or a virtual piece of scrap and yet weigh the same amount. You say the postage is $4.

That is really cheap. My daughter recently posted off a concert ticket. You know how light those things are, a mere wisp of paper. The postage she charged on that was around $6 or $7 mark, from memory (sorry, I can't recall the amount exactly). It went in a perfectly flat envelope. But the reason it was that price was it had tracking, the person receiving it would have had to sign for it. She got positive feedback from her buyer.

I am guessing if you had had to pay for tracking, postage might have been even dearer.

I do agree with you the seller should have go back to you way before 3 days, that is slack.

And it would not have killed them to combine postage or perhaps even charge you the same amount but upgrade it to tracking for you.

Since they let you off the purchase though, is it really a case of them deserving a neg? After all, you are the one who bid, knowing what their postage cost was. You took a gamble there would be combined postage but that isn't their fault.

In fact, I buy fom some daily deal sites and in plenty of cases, they don't combine postage for me either when I buy two or more items. So it isn't a 'given' it will happen.