Depending on which font you want to use there are different solutions. As @Supporter said, if you go to Settings, then Shipping and Delivery, then to Packing Slips Template, you’ll see all the HTML, Liquid, and CSS which make up your packing slip.
If you click into that code box and use the search keyboard shortcut (ctrl+f or cmd+f on Macs), you can then type in “family” and the screen should jump to the font-family tag in the CSS section.
You can remove the code between the : and ;, then test it out with the most generic common font families: serif, sans-serif, monospace, cursive and fantasy.
Should look like this: font-family: cursive;
Save that, preview, and all of your text has hopefully changed. Right? (It might not look nice, but you’ve got something to change and that’s a beginning)
Now, you have two options (more or less), you could use one or more of those fonts in whatever combination looks best. A common idea is to use serif for titles/headings and sans-serif for the rest of the text. It might not look bad.
The easiest way to do that is to have the “wrapper” (as mentioned before) and change the font-family to sans-serif, then search for “.to-uppercase” and add the line like this:
.to-uppercase {
text-transform: uppercase;
font-family: serif;
}
Save that, and preview. You can now pretty much play around with fonts. The problem here is that Shopify only recognises certain fonts…
The workaround for that is pretty involved, but there is one here: Google fonts suddenly not working on packing slips
Let me know if that’s any help, or if I haven’t written things well.