thanks for looking into this.
I created a test project with the latest version of your plugin (126.96.36.199) and sharing it. included cocos 4.0 to be faster for you:
1 – download the sample project from https://1drv.ms/u/s!AiuQ2l1EMumQicQtmitBG_JHAUfLSw?e=GSxoMz
2 – open proj.android/gradle.settings and fill the RELEASE_STORE_FILE and the other key related properties. add the correct JDK path to org.gradle.java.home
3 – fill your app package name on the manifest file
4 – open proj.android/app/build.gradle and fill your applicationId with the package name you entered on the last step.
5 – fill your iap item details on sdkbox_config.json
6 – on the helloWorldScene.cpp fill your IAP item name
7 – build and install with gradlew installDebug
8 – buy and item and try to buy it for the second time.