How to enable Cart label/line item properties for your store?
Shopify provides instructions on adding support for line-item properties to your theme in their documentation: Product Pages - Get Customization Information For Products
This process involves modifying your theme's HTML/Liquid files. If you're not familiar with coding, it's advisable to seek assistance from your theme developer.
How to display this properties on your invoices?
To display line-item properties on your invoices, you'll need to modify your shipping label templates. Refer to Shopify's documentation on line-item properties for detailed guidance: Line item properties.
Again, this task involves technical aspects like HTML/Liquid coding. If you're not comfortable with this, consider asking your web developer for assistance.
What's the maximum length a pre-order cart label can be?
Pre-order cart labels can be up to several paragraphs in length, however we recommend keeping them as short as possible.