First of all, your blower isn't removing "thatch". It's removing grass clippings and anything else that gets mashed into the lawn. Removing thatch takes a machine that cuts vertically into the soil where the actual thatch layer is. You are merely power raking the lawn.
Secondly, you don't just add services and expense before consulting with the customer do you? Try to sell them the additional work and why it would be beneficial to have it done. I personally wouldn't just do the extra work and then expect to have them pay for it. It's a tight economy and that would be a good way to piss the wrong person off on the wrong day.