There is a new dropdown and button for selecting your desired UI solution in the Project Settings - making use of this goes over all Simple IAP System script files and e.g. replaces Unity UI references to TextMeshPro (Text > TextMeshProUGUI). Currently also InputField > TMP_InputField.
It is easily extendable with more classes or even a completely different UI asset, you just have to maintain the source and target types in the UISourceConverterData script. Note that the scenes included still make use of Unity UI, so after converting all references to TextMeshPro they will not work anymore. The converter is designed for when you are using your own shop scenes, or did the conversion manually already, but want to upgrade Simple IAP System at some point. You could basically do a "Replace All" in Visual Studio for that, but it is built-in now with that option.