ebay invoice

That might depend on the type of eBay account you have.
When I first started, I did not have access to any sophisticated tools.
What I would do is send one invoice that specifies the two items. There should be a spot to add additional charges. Also, there is a message window to explain what you are doing. Then, just cancel the other invoice. If you maintain good communication, that should work.

I made an invoice earlier today for them sent it. when I tried to make a new one it said the exact same amount and everything...

Perhaps it's time to call support.