Customer service is good for simple, common account issues where the representatives can look up the answer and follow the script they are provided with. It is not necessarily suitable for uncommon or difficult problems that require extensive troubleshooting.
If you are having a problem with your cart, remove all the items from your cart and avoid using the cart. Purchase items individually rather than adding them to the cart. Wait a while before attempting to use the cart again to see if the problem goes away.
There may be an issue with your account that is causing you to receive an error message -- apparently the wrong error message, as the problem is not related to the sellers being on vacation if you are still able to purchase from those sellers. Perhaps try a different method of adding an item to your cart. If you get that error while using a browser, try using the eBay app instead and see if you get the same error. If you get a different error, that may allow you to figure out what the actual problem is. If you are able to use the app to add items to your cart and check out that way, you will have at least found a workaround you can use, though I believe that the app lacks the feature to request an invoice. The app may work to combine items, though, if the seller has set that up properly.
If none of that helps and you must purchase multiple items from the same seller and you need to use the cart to request an invoice or to combine shipping, try signing in to ebay.ca and adding your items to the cart there. Each country site has its own cart system, and they are not connected; perhaps that will work better for you than the cart on ebay.com.
Your account is relatively new; it might be worth registering another eBay account and seeing if that allows you to add multiple items to your cart, check out and bypass the problem that way.
