How do I enter invoiced mileage allowance as an expense?

Yes, travel expenses are indeed turnover. You can, in other words, ask for whatever you want here, as long as your customer agrees. I think you are a bit confused with the mileage allowance. That is a completely other principle for entering your car expenses. I think Dexxter has an article about this. It's worth searching for it in the learning center. I also had my doubts about this, but I feel that my actual costs are higher than the mileage allowance.

If you want to record your fuel purchases, you will need a receipt, otherwise you will have to exclude it from your accounts this time.

• Stef