Hi Elouisa - I don't think I've ever seen a theme for an invoice that allows you to section off expenses you recharge on an invoice. What most people do is to just manually add an invoice item and add the expenses one at a time or in bulk as one entry.
If there are specific expenses that you have spent out that you want to recharge then you can do this via Purchases>Expenses>Recharge, this lists all your expenses and costs, you select the ones you want to recharge, hit the "Add to invoice" button and a sales invoice is generated with the expenses pre loaded. If you can't see this option on the Expenses menu then switch it on in Settings>Toggle features>recharge costs.
Come back if anything not clear and, with regard to the software being complicated, it has a lot it can do but you should soon get the hang of the basics and we are always here, either on this site or via support to help. Also have a think about some training if there are bits that really baffle you.
All the best