As a travel agency, they're required to tax the profit, i.e. the service charge, gained from selling air tickets, hotel bookings, tour, etc. Basically:

• (tax) = ((sale price) - (cost price)) * (5/100)

There are two problems:

• calculating the tax based on the difference of sale and cost price

• setting the cost price when creating invoice

The first one is self-explanatory. So far, I've only seen ability to set tax based on price of the product, not the associated service charge.

The problem with the second is, even if it's a flight to the same destination, the cost price varies from day to day, yet there's no way to let Odoo set the cost when creating invoices. On the other hand, it doesn't make sense to create a new product for every invoice either.

What can I do? I'm open to changing the database models and introducing new form entries, but what exactly must I modify to resolve the two issues?