Try another browser to rule out browser issues. Preferably the latest version of a popular browser with default settings and no add-ons installed. Or the eBay app.
Not all items can be combined into one transaction. Try paying each seller separately if there is more than one. Or purchasing single items from the individual listing.
Check to see if any of the items appear in your Purchase History as transactions awaiting payment; if so, you may be able to pay there.
If you have not yet committed to those purchases yet, try checking out on ebay.ca instead of ebay.com.
