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I think there are two ways of approaching this problem (people changing their minds after the fact). Although I agree with 'femmefan' that it's a good idea to weed out the non-payers by filing Unpaid Item Disputes, the question is -- how do you define a non-payer?
This is something you have to decide for yourself, to fit within your "comfort zone" of selling on eBay. I'll tell you what my approach is. You may agree or disagree, but it's the way I personally deal with such issues.
If a buyer purchases one of my items and then essentially vanishes for 4 days (no explanatory email, no payment, no request for cancellation), then I send one friendly reminder, wait 2 days, then file an UID. In nearly every case the buyer has finally paid. Is that a non-paying customer? I don't think so, strictly speaking. In such a situation, I'm not sure I've made a happy buyer (who might be back the next time with more money).
If a buyer purchases an item then sends me a message saying her dog hit the "Buy Now" button and she doesn't want the item -- or whatever the excuse -- then I'll send a message back very politely asking if the buyer would like to cancel the transaction. If I get a message back saying "yes, please", then I will process the cancellation using the reason "Buyer made a mistake or changed their mind". You will not get a defect for that sort of cancellation.
I usually send a message saying I'm sorry the transaction didn't work out for the buyer this time, but hope she'll visit my store again. In that scenario, I think I've likely made a happy buyer by letting her off the hook entirely, with no consequences for her or for me (and a buyer who might be back the next time without the dog).
Now you may think I'm making myself into a patsy doing this. However, think about all those times -- especially if you're in a hurry or doing Christmas shopping for example -- when you've picked up an item in a brick-and-mortar store, put it in your cart, looked at it, put it back, picked up something else perhaps, then changed your mind again, perhaps at the last minute just as you're in the cashier line. I know I have. And I certainly wouldn't want one of the store clerks to be chasing after me forcing me to pay for something I really didn't want.
In the end, I think it boils down to how comfortable you are with accommodating reasonable requests. My own "line in the sand" is if people don't contact me at all, i.e. don't even make an effort to think up an entertaining excuse. Then I think they deserve the UID. Those are the ones I will block after the dust settles.
Some just decide after the fact that they purchased in haste and wish they hadn't. Those are the customers I have no problem accommodating. If they think I was fair to them when they needed it, then my feeling is that they might be back.
I do agree with 'femmefan' that mobile devices, combined with mass-A.D.D. these days, are probably going to cause more and more of these "whoopsies". Still, I'd rather agree to a pleasant buyer's request to cancel and be able to re-list as soon as they accept the cancellation, than have to wait through the UID process to relist. Keep in mind that a buyer who does pay after a UID is opened can also leave feedback, so continued courtesy is in order throughout the process.
I took a look at a couple of your items at random. Is there any reason why you are using calculated shipping on some and flat shipping on others? Personally I prefer flat shipping because the buyer sees the shipping cost right up front, immediately, no further muss or fuss required. I think 'femmefan' may be right that some of your problematic transactions might have resulted from buyers not being able to see shipping costs before buying.
By the way, I tried entering my (Canadian) postal code into your shipping calculator and got the message "Shipping cannot be calculated". I suspect something is wrong with the sizes and /or weights you're inputting. That's one of the little irritations about calculated shipping I don't like. You really have to know and study your Canada Post rate/size/weight tables, both for inches and centimeters. Sellers have reported issues around the dimensions not working out properly -- I don't use calculated shipping, so you'll have to ask someone who does what the problem and the solution is.
I find flat shipping works better for me because not only do my buyers see the shipping cost immediately at the top of the listing, but I also have direct control over what I charge for shipping. This means I can "subsidize" some of the rates in order to attract customers and save on FVFs. I know many sellers prefer calculated, but the concept has never appealed to me.
You are probably selling in a category that will attract younger, less attentive or more impulsive buyers. You need to do everything you can to present all the relevant details to them in one easy step.
And yes, I would seriously revise the "PAY ATTENTION TO THIS" commentary below your listings -- at the very least soften the language, and say "Please Note", or "To My Valued Customers", etc. at the top (and please, not all in capital letters, it looks like nasty shouting).
You have some interesting items for sale. Best of luck to you! 
eBay says that 62% of all transactions are "touched" by a mobile device. eBay also states that in excess of 50% never read the description.
They type in a couple three keywords and then choose the best picture. Yeah, they like to decide on the best pictures. Include more pictures than you think is reasonable, Get very good at pictures as the pictures do the selling.
All of my listings are IPR. Some sellers do not use IPR because it drives away people that do not want to pay immediately. At the end of the day, you should still have the same number of paid for sales. My ditherers simply never make the attempt to buy, yours do.
I have come to believe in either flat rate shipping or "free". In the last 18 months, the Loonie has dropped 30% against the Greenback. This has allowed me to compete head to head with American sellers. I buy in Loonies and sell in Greenbacks. Compared to Dec 2012, three years ago, I am making about 43%.
Do not look at shipping as "extra", instead, it is part of the cost of the widget.
I do not play the race to the bottom game, I compete with high price sellers. That works very well for me.
I have tried all sorts of different combos, and, for teh last year, BIN with IPR and "free" shipping is working fantastic for me. I am free to the US, Canada, and depending upon weight, about 98% free to worldwide.
Get rid of that "Please pay attention here" as that is rude and condescending.
50%+ buyers never look at or read the description. Use flat rate or free shipping.
