A custom computer program by the MoF definition is "designed and developed to solely meet the specific requirements of, and that is intended for the exclusive use of, a particular person". The sale of a custom program is only exempt to the original purchaser. It is best to sell such program using a sales contract or agreement to document the charge is for the purchase of a custom program. Then and only then is it exempt, otherwise, be safe and charge the tax. Have you accountant check the TAX FAX service at 1-877-482-9329 and look for guide 650 published February 2001. Good luck!