Don’t know if op is still looking for help, but I found this question when I was looking for the answer myself and wanted to post the answer for other people looking for it.
I don’t know HTML, and Shopify helpfully suggested I borrow money from them to pay an expert for one line of code. Luckily, I am related to a coder who fiddled with it until we got it working! I think Shopify’s default packing slip code is the same regardless of theme, so hopefully this will work for everyone.
Replace the line
{% for line_item in line_items_in_shipment %}
with these lines
{% assign sorted-list= line_items_in_shipment | sort: “title” %}
{% for line_item in sorted-list in line_items_in_shipment %}
Hope this helps someone else!