For what it's worth, I don't think you come across as an unreasonable or troublesome buyer (and I am speaking in my capacity as a seller, rather than empathising as a buyer - you'd be welcome at my store any time).
To me, it sounds like your attempts at contacting the seller springs from concern over leaving the item unpaid, and subsequently concern over not receiving responses as promised by the seller, which I personally think is understandable. Ringing a few times over the course of the day on which the seller instructed because there was no response is not something that indicates 'trouble' to me, I call people back a little later if they don't answer the first time, too....
In saying that, generally, these days I'd make a couple of attempts at contact only, spaced over the course of a few days, and then drop it, though I may be tempted to be a little less patient if it was something I really wanted, as then I wouldn't be able to put it down to 'seller's loss' 
. The reality is that there's only so much you can do to get a seller to do what they need to on their end, so the only reason I send more than one message now is just in case the first one didn't go through (which happens from time to time with eBay messaging). It's clear that the seller's lack of response is an unexpected anomaly in so far as your experiences to date, and even if your attempts to communicate with them could be deemed harassing, all I really see is someone whose primary concern is being able to pay for the item - though you've mentioned being frustrated, and that this is concerning, on the whole you haven't said or done a thing that would concern me as a seller.
And, as PJ mentioned, if the item is being posted, don't pay without a new invoice with a postage quote.
At this stage I would presume that to mean that the seller still has not provided a new invoice with a postage quote (auction items do appear in the cart, and basically eBay wouldn't know that you're waiting for a postage quote). When items are in the cart have a postage option, it usually displays the method the seller selected (eg "standard", "Australia Post Parcel" etc).
If the seller has sent an invoice, there should be an email notification / copy from eBay, so I'd check for that first. A lot of sellers, when they make changes to postage prices, methods etc, hit a button labeled 'recalculate' thinking this applies their changes, but it doesn't (rather, it reverts the invoice to the original), so if you do have an invoice and nothing has changed, it may be due to that.
I'd almost be tempted to send them one last message and say something like "Thanks for sending through the invoice for shipping, I didn't expect free post but it's certainly very much appreciated - I'll send payment this evening".
If they are around, it might actually prompt them to get in touch so they can correct you. 
(Though, obviously if they're absent for another reason, that won't work, nor will it if they indeed meant to provide you with free shipping, so maybe a 30% chance or so of actually getting you the details you need).
