By default, purchase action s trigger when someone purchases through the shopping cart and order forms.
mPOS.sale uses API to create and process your orders. Hence, you need to enable an option in your Infusionsoft account to make sure purchase actions are triggered for API orders too. If you are using other API tools available in market, there is a good chance that this setting is already configured in your account.
Under E-Commerce > Settings > Products section, there is an option titled "Run Purchase Actions On All Orders". That option needs to be set to "YES".