It is possible you don't have a package option large enough to (in theory) fit the sizes of the two items, according to the dimensions you provided Etsy.
I have a similar thing happen sometimes; what I did was make the larger package option really a lot bigger than it is (it doesn't affect the price to ship, so that might be a concern for you depending on your package sizes) so that Etsy always thinks everything can fit in the one package.
For this order, you can just put it all in one package and refund the extra shipping cost the buyer paid.
