Automator : How to add a specific file to an Automator folder structure every time

Is that always the same excel file, from the same location?

Try the "Get Specified Finder Items" action. This can easily get that excel file into your Workflow.

Or just add the appropriate shell command to tou your shell script.

cp <path-to-file> <destination>

for example, it may be

cp ~/Documents/myexcelfile.xlsx "$1/$1/Documents"

Whenever you type commands in Terminal (or a shell script in your case), you have to be mindful of spaces, as they are used to separate things.

Therefore

Costing Sheet

is read as two separate words.

Renaming to avoid spaces is a good idea, as it can steer clear of a number of pitfalls; but in this case an easy solution is just to enclose in double quotes, e.g.

cp "~/Documents/Costing Sheet.xlsx" "$1/$2/Documents"

You may have noticed that already the arguments of your mkdir commands were enclosed in double quotes; this too is necessary, in case the paths to those folders have spaces in them.