Hi, Philip!
It sounds like what you’re looking for is the Formatter “Line-Item to Text” action, as it can take multiple line-items from your Shopify trigger, and combine them so that each line from the order appears as a line in the card. You can read more about the action here:
https://zapier.com/help/create/format/convert-line-items-into-text-strings
Specifically, you’ll want to map each of the values you need into the “Input” field, and set the “Separator” field to [:newline:] to add a line break between each. Here’s an example using product name, price, and quantity:

(open image)
That will give you a few different output values, but “Text” will be the one you’ll want to use in Trello, as it will be each of your line items, separated by a new line:

(open image)
That way, you can have nice clean lines in your Trello card.
I hope that helps!