Red envelopes are widely used in the Black Sect Tibetan Tantric Buddhism School of Feng Shui (BTB) as a means of respecting and protecting the transmission of the ancient feng shui knowledge.
Usually, the BTB feng shui practitioner will ask for several -- it can be 3, 7, 9 or even 21 -- red envelopes with some money in them, given at the time of the feng shui consultation.
It is interesting to note that there is an expression, or tradition in China called "the red envelope policy" which says that one has to give a bribe to get what he or she wants. Red is a strong color that "burns" or prevents negative Chi from expanding so it is widely used for this transmission.
We also know that the red envelopes filled with money or little gifts are commonly used for Chinese New Year, weddings, birthdays or other important events.
To sum it up, the red envelope tradition in feng shui is a means of protecting the ancient knowledge, as well as showing respect for the feng shui consultant who shared this knowledge with you.
Ask your feng shui consultant if he or she requires the use of red envelopes. To thank for the feng shui consultation, you can use any red envelope if you do not have access to traditional Chinese red envelopes. Do not give them empty, as they have to contain some money in them. Of course, this is in addition to the actual payment for the consultation, and not instead of the payment.