Mileage allowance and fuel costs

Ah yes, income via the sales invoice.

Then enter the expenses to reduce the profit.

Since you are self-employed, you can decide for yourself what you charge the customer. It is best to make clear agreements about this before you start the job. I charge a mileage cost (that is the cost of your car: depreciation, maintenance, fuel, etc. you can google the price for this) + the hours I am on the road. Some companies work with a flat-rate "call-out fee". It depends a bit on the sector you work in.

Quite right, 'BD'. Ultimately, you decide the mileage allowance you charge your customers. It doesn't matter, it's all turnover.

But on the purchasing side, you can enter a fixed amount, which is limited.

On the purchasing side, I enter all expenses in the cost category 'light cargo fuel/maintenance'. I don't know how you can work with a flat rate on the purchasing side.

You should search for 'mileage allowance' in the learning center ;)