How do I show text on a packing slip only when an order has a specific tag?

Overview

Custom HTML packing slip templates (and Sales Order document templates) can now read an order's tags. You can print all of an order's tags, or show a block of text only when the order carries a particular tag.

Add a tag-based block

Go to Manage > Templates > Packing Slip and click Edit.
In any HTML field, click Insert Placeholder.
Open the Order Tags group, or search for "tags". Each active tag in your account is listed as tags.<key>.
Insert the entry for your tag. It looks like this:

{{#tags.engraving}}NEEDS ENGRAVING{{/tags.engraving}}

The text between the opening and closing markers prints only on orders that have the "Engraving" tag. Orders without the tag print nothing there.

Print all tags

{{tags}} prints the order's tag names, comma-separated (for example "Engraving, Gift").

How the key is built

The key is the tag name in camelCase with punctuation removed. For example, "Gift Wrap" becomes giftWrap. If two tags produce the same key, they share one picker entry that names both tags.

Check which keys an order has

Put {{{tags._info}}} in the template and do a test print. It shows the tag keys on that order.

Accounts with more than 500 tags

The picker doesn't list individual tags in this case. It shows a note instead. Type the {{#tags.<key>}}...{{/tags.<key>}} block by hand, using the key rules above.