Thank you for the opportunity to clarify.
Unity IAP is a split product. Both packages are needed for a full installation of the Unity IAP SDK, currently. This will be changed (see below).
Currently:
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(FIRST COMPONENT) The “package”, a.k.a. the “Package Manager package” com.unity.purchasing, a.k.a. “In-App Purchasing”, a.k.a. the “core API”
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(SECOND COMPONENT) The Plugin, a.k.a. the Asset Store package, a.k.a. the “Unity Package”, a.k.a. Unity IAP
Surprisingly, and confusingly, both components have identical version numbers as of this month. [An illustrative aside, if I may use an analogy, like when the moon passes in front of our sun, it is also a surprising moment for the Unity IAP components’ versions to become identical.]
To deepen the explanation for this split Unity IAP’s core API (now the package) was shipped in the Unity Editor binary, in Unity 5.3 → 2017.1 … for several years. The team shipped updates for app-store bugs and features through the plugin via the Asset Store. And after 2017.1 we trivially converted the core into a package. Now with Unity’s Package Manager being fully featured for quite some time, we are preparing a merged version of Unity IAP that simplifies installation (see below, again).
Another aside, for your information, is the core API contains enough code to allow a developer to use its “IStore” and “IPurchasingModule” APIs to develop a new app-store, e.g. for an existing or new in-app purchasing service. Like if there is a need to support a hypothetical Jordanian mobile network’s in-app purchasing system in your app, not currently supported by Unity, then implement IStore & IPurchasingModule, and use the same Unity IAP purchasing API to make purchases there!
For Unity IAP v3.0.0, we are merging to just one, com.unity.purchasing. Please see the merge discussion .