Top left side of your Desktop screen where it says "Hi (your name". Click that dropdown. Go to account settings. You need both the business policies and shipping preferences tab. You use shipping preferences to create flat rate shipping rules (Buyer pays x amount for each extra item) - you can then go to business policies and create a shipping policy that has $3.45 shipping where each extra item costs x amount on top of that.
You might have to play around with these two functions to get it to work the way you want. It's not very intuitive. The best way to test it would be by making a few Buy It Now listings with the shipping preferences and business policies, and then adding them to a cart in an incognito window where you aren't signed in. You can put in a Canadian address, and a US address, and see what combined shipping rates it spits out at checkout. You don't have to complete the checkout, you just want to make sure that it's actually charging combined shipping how you want.
There are a few considerations here. If you're shipping lettermail to the USA, while you might get away with it sending a card without a toploader in a PWE, it's not supposed to be allowed and card sellers do get returns. By law all imports to the USA need a CN22 form. So you need to use parcel shipping. Another thing to look at is that if you're paying $12.21 to ship 3 cards to the USA, you can pay significantly less using eBay labels. A 99 gram tracked packet label is around $7.
On the topic of your auctions. It's two things. Yes, it is the high price that is stopping bids. But looking at a few inserts that you're selling, they really aren't worth enough to sell individually unless you're leveraging volume like crazy (as in you're a full time seller packing dozens of orders daily) and can list+pack orders very quickly to the point that make 25 cents to a dollar from a sale is worth it when accounting for the cost of your time.
Most will sell for $3-$6 shipped BIN. Auctions almost always sell for less tha BIN and don't realize full value. Thats why your auctions are ending with so little interest. I don't think $3.45 CAD shipping to the USA will stop this. I sell media, and it's a very similar thing. There might be a handful of people in the country at any given time who wants a specific CD or DVD. BIN, it might sell for $15-$20 within weeks or months, if I were to auction it for 75 cents it would sell for $2-3.
Given the type of items you're selling I would consider coming up with a reasonable ceiling for shipping (probably $4 in this case) - and selling your cards via BIN or Best Offer with the shipping bundled into the BIN price. Or, I would look into a platform like COMC, which might be a better fit in terms of the time/effort/cost of unloading these inserts.
Just to illustrate why I think BIN at a higher price would be better, you're selling cards for 75 cents. If someone in the US wins your card, you're paying them to take your card. The reason, eBay fees. eBay is going to charge you 13.5 percent or so on the entire total of the auction. So even if someone in Canada buys your card for 75 cents, plus $2 shipping, plus as much as 15 percent sales tax, eBay is going to take roughly 50 cents from that. If someone in the USA manages to buy your card for $12.21 shipping, eBay is going to charge you close to $2 in seller fees. So in that scenario, you're paying someone to take your card.
