Various technologies and techniques are disclosed for performing tax calculations. An add-in model is described for developing add-in(s) for performing tax calculations. The add-in(s) calculate taxes for one or more tax authorities for items contained in a transaction being processed by a point of sale application. The add-in model specifies functionality the add-in(s) need to implement before the add-in(s) can be used by the point of sale application. A method for configuring multiple add-ins is described. A first add-in is called to calculate a first partial tax owed and the first partial tax is received back. A second add-in is called to calculate a second partial tax owed and the second partial tax is received back. A total tax is calculated by adding the first partial tax with the second partial tax. A configuration user interface is also provided by the add-in.