Hey there,
So best thing would be a function node to loop the items and do the maths as well return anything else you wanted to post.
Take a look at this thread for an idea: My first function? - #3 by RicardoE105
On Monday what I will do is clean up my Zendesk to Slack workflow and submit it as an example to help with questions like this, while the data source is different the theory is the same.