Is a programmer sinful if he designs an accounting and invoicing system for a shop that sells smoking accessories and electronic cigarettes, whether it is a ready-made system or a new one that will be sold later to parties other than smoking shops?

It is not permissible to sell systems and programs to shops specializing in prohibited items, such as tobacco, because doing so constitutes direct assistance in sin. Allah Almighty says: "And do not cooperate in sin and aggression." The one who assists and causes is a partner with his companion in the consequences of his action. As for shops selling food, medical supplies, and permissible items, there is no harm in selling systems and programs to them. The decisive factor is how the program or system will be used.