In some categories ebay requires that a buyer pay before the item is actually reserved for them. It doesn't look like your cards are in that category though. But to confirm that...are you buyers being asked to pay as soon as they click on BIN or is the problem that they have already used BIN for more than one item and don't know how ask for a combined invoice?
Normally when a buyers buys more than one item they will see a link that they can use to request an invoice from the seller. When the seller receives that request they can combine the invoice and change the shipping if needed. It sounds like either that link for 'request an invoice' has been moved or for some reason it no longer exists. I don't have any current purchases so can't look to see if there is a link somewhere but perhaps someone here knows where it is.
As far as a cart...yes other sites do have a cart but other sites don't have millions of sellers who charge different shipping costs. I've heard that the cart on .com has or has had quite a few glitches so perhaps that is one of the reasons why it isn't on .ca.
If your buyers want to use the cart, they can simply sign into ebay.com to use it. It doesn't matter which site the listing originated on.
Another suggestion...it looks like most or all of your listings are some items with the same weight and shipping cost so it should be fairly simple for you to set up some combined shipping rule so that if like items are purchased from different listings, the shipping rule should automatically combine shipping.You can set up the rules under Account - shipping preferences - offer combined payments and shipping - edit
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