Your customer isn't paying for the code you write, they are paying for a finished product that works. How much code you write should be irrelevant to them. In

Your customer isn't paying for the code you write, they are paying for a finished product that works. How much code you write should be irrelevant to them. In fact, one could argue it is far more dishonest to quote without testing since that does not include the maintenance costs that will be incurred by them after the initial product delivery, which will almost certainly be higher than a project with automated tests.