The file already has a newline at the end, terminating the final line. Anything append to it, therefore comes on another line. You'll need to overwrite the final newline, which you can't do with just a
>> redirection. Probably easiest to use
$ seq 1 4 | sed -e '$s,$,stuff/to/append,'
sed modify a file in-place, use
sed -i -e '$s,$,stuff/to/append,' filename
(The dollar signs have two meanings there. The first
$ is an address that controls what line the next command runs, here it means the last line. The second
$ is regex notation for the end of line.)
To append to some other line than the last, replace the first
$ with either a line number or a pattern match. This would append a string to any line containing
sed -i -e '/foo/ s,$,stuff/to/append,' filename
Note that I used commas here as separators for the
s command. Slashes would be more common, but the text to be added contains them. The
s command allows (almost) any character to be used.
If you have slashes in the pattern used to find the correct line, you can use something like
\,foo, as the address, i.e. start with a backslash and use whatever separator is convenient.