Is it possible to create an invoice that doesn't show line items cost breakdowns, or do I need to follow this guide and create a second template for this customer? https://galaxy.maropost.com/s/article/set-up-invoice-templates-for-specific-customers
It depends what you're trying to achieve. Cloud Commerce is super flexible with the documents you can create.
This is for a specific customer that wants just a total invoice at the end of a month not an item by item breakdown when a month's worth of invoices are merged into one.
We will still use a standard invoice template for everyone else.
You'll definitely be best off following the steps in that article then, should do exactly what you want. You'll need some HTML knowledge to be able to edit the templates but if all you want is simple changes like the price removed you should be able to find the right parts to delete.