@Paparazzi, I suspected you were doing something like this, but did not know what you were selling. You can do exactly what you describe with non-inventory items and have the account changed automatically. The example in my point #2 is similar.
Define a non-inventory item under settings. Specify the income and expense accounts you want to use. Create a purchase invoice including the non-inventory item, and it will be posted to the expense account you chose. Copy the purchase invoice to a sales invoice, and it will automatically be shifted to the income account you designated. You will, of course, need to specify a customer, because the supplier no longer is relevant.
If you want, you can also specify purchase and sales prices, although those might not be constant for you. But if you do, they will change automatically, too. For example, if you set up your non-inventory item as 1 km of transport, purchased for 10 AED and sold for 11 AED, you would make 10% profit on every km sold. As you enter a trip, you would specify the number of km involved.
You would actually get even stronger guarantees against accidental mistakes than with the method you suggested. And if you have different rates for different situations, you could set up additional non-inventory items.