Not sure if this belongs in #C29HQS63G or #C466X49JB so I will ask in both. I am trying to make a custom pdf which will essentially be identical to a packing slip but it will be get the data from a few item fulfillments instead of just one. What is the best way to go about doing this? I see in the help center that I can create a advance pdf template and populate the data from a search. Is this my best option?

depends on if you're using the render module - renderAsPDF and with custom data sources, easiest way is if you're using this as a scripted solution, you can use some tag which is a comment on a saved xml, say <!--replace this from fulfillment--> and then you can use javascript to replace this tag