Warning about eBay International Shipping (EIS): No Combined Shipping
I've spent a ton of time on the phone with eBay today and yesterday trying to figure out an issue, and after some more digging today, it seems many other sellers have run into this. In short, if eBay International Shipping is enabled and a buyer wins multiple items, they cannot be combined, nor can you send an invoice to change any settings (such as shipping internationally yourself and not using EIS).
Beyond this, eBay isn't even consistent. A buyer won two coins from me (both Canadian, one in each of the two Canadian 5 cent categories) and one got sent down the EIS path and the other looks like a regular international sale. I can send an invoice and modify the shipping options at will for the latter, and I can't do anything for the former. It's a huge mess. I decided to give EIS a go not that long ago, and boy am I regretting it now. I've turned it off on my account and will go back to always handling international shipments on my own, as I've been doing for the last two decades or so.
If you want to disable EIS (or make sure you already did) the setting is on this page: https://www.ebay.com/ship/prf (or navigate to it by hovering over your name in the top left corner of eBay.com, clicking Account Settings, then Shipping Preferences in the bottom left column under the bold Selling header).