xargs sed and command substitution in bash - bash

Question

I'm trying to pass a xargs string replace into a sed replacement inside of a substitution, here's the non-working code.
CALCINT=$CALCINT$(seq $CALCLINES | xargs -Iz echo $CALCINT' -F "invoiceid'z'="'$(sed -n '/invoiceid'z'/s/.*name="invoiceid'z'"\s\+value="\([^"]\+\).*/\1/p' output.txt))
Everything works up until the sed inside the second substitution. the 'z' should be a number 1-20 based on the $CALCLINES variable. I know it has something to do with not escaping properly for sed but I'm having trouble wrapping my head around how sed wants things escaped in this situation.
Here's the surrounding lines of code:
curl -b mycookiefile -c mycookiefile http://localhost/calcint.php > output.txt
CALCLINES=`grep -o 'class="addinterest"' output.txt | wc -l`
CALCINT=$CALCINT$(seq $CALCLINES | xargs -Iz echo $CALCINT' -F "invoiceid'z'="'$(sed -n '/invoiceid17/s/.*name="invoiceid17"\s\+value="\([^"]\+\).*/\1/p' output.txt))
echo $CALCINT
Output: (What I get now)
-F "invoiceid1=" -F "invoiceid2=" -F "invoiceid3=" -F "invoiceid4=" -F "invoiceid5=" -F "invoiceid6=" -F "invoiceid7=" -F "invoiceid8=" -F "invoiceid9=" -F "invoiceid10=" -F "invoiceid11=" -F "invoiceid12=" -F "invoiceid13=" -F "invoiceid14=" -F "invoiceid15=" -F "invoiceid16=" -F "invoiceid17=" -F "invoiceid18=" -F "invoiceid19=" -F "invoiceid20="
What I'm hoping to see as output is something like this
-F "invoiceid1=2342" -F "invoiceid2=456456" -F "invoiceid3=78987" ...etc etc
-------------------------EDIT-----------------------
FWIW...here's the output.txt and other things I've tried.
for i in $(seq -f "%02g" ${CALCLINES});do
sed -n "/interest$i/s/.*name=\"interest$i\"\s\+value=\"\([^\"]\+\).*/\1/p" output.txt > output2.txt
done
output2.txt contains nothing
Thanks to #janos response for clearing things up but taking a step back makes it clear to me that the root of the issue here is that I'm struggling to get the invoice ids out. It's dynamically generated HTML "....name="invoiceid7" value="556"..." so there isn't anything consistent in those particular tags that I can grep on, which is why I was counting another tag that IS consistent then trying to use a variable sed to basically deduce the tag name then extract the value.
Annd..output.txt https://pastebin.com/ewUaddVi
------UPDATE-----
Working solution
Stuff sed into a loop. Note how I had to use ' to use variables in the sed string. That is well documented elsewhere on here. :)
for i in $(seq ${CALCLINES});do
e="interest"$i`
CALCINT=$CALCINT' -F "'$e'='
CALCINT=$CALCINT$(sed -n '/'$e'/s/.*name="'$e'"\s\+value="\([^"]\+\).*/\1/p' output.txt)'"'
done
Please read through the comments on the solution below, there is a cleaner way of doing this.

Answer 1

Your current approach cannot work, specifically this part:
... | xargs -Iz echo -F "invoiceid'z'="$(sed ...)"
The problem is that the $(sed ...) will not be evaluated for each line in the input during the execution of xargs.
The shell will evaluate this once, before it actually runs xargs.
And you need there dynamic values from your input.
You can make this work by taking a different approach:
Extract the invoice ids. For example, write a grep or sed pipeline that produces as output simply the list of invoice ids
Transform the invoice list to the -F "invoiceidNUM=..." form that you need
For the second step, Awk could be practical. The script could be something like this:
curl -b mycookiefile -c mycookiefile http://localhost/calcint.php > output.txt
args=$(sed ... output.txt | awk '{ print "-F \"invoice" NR "=" $0 "\"" }')
echo $args
For example if the sed step produces 2342, 456456, 78987, then the output will be:
-F "invoice1=2342" -F "invoice2=456456" -F "invoice3=78987"