There has been very limited experimental evidence that specifically discusses small businesses’ tax compliance from the lens of behavioural economics. This study investigates the compliance behaviour of small businesses in Indonesia by utilising randomised controlled trials conducted in collaboration with the Indonesian tax authority—Directorate General of Taxation (DGT). The study analyses the tax compliance effects of three treatment letters, including deterrence, literacy, and public goods provision letters. The paper finds that all treatments significantly increase tax filing and payment. The deterrence letter has the most significant effect on payment. The literacy letter triggers the largest increase in filing rates and timely filing but the lowest increase in payment. The paper also finds a considerable impact across distance, internet accessibility, and compliance history, which provides a fuller picture of small businesses’ compliance behaviour considering perceived behavioural controls, transaction costs, and risk aversion.