Retrieving available items

First thing you should do is to define your product IDs for iOS and Android separately like defined below.

import * as RNIap from 'react-native-iap';

const productIds = Platform.select({
  ios: [
    'com.example.coins100'
  ],
  android: [
    'com.example.coins100'
  ]
});

To get a list of valid items, call getProducts().

You can do it in componentDidMount(), or another area as appropriate for you app.

Since a user may first start your app with a bad internet connection, then later have an internet connection, making preparing/getting items more than once may be a good idea.

Like if the user has no IAPs available when the app first starts, you may want to check again when the user enters your IAP store.

  async componentDidMount() {
    try {
      const products: Product[] = await RNIap.getProducts(productIds);
      this.setState({ products });
    } catch(err) {
      console.warn(err); // standardized err.code and err.message available
    }
  }

Each product returns from getProducts() contains a Product object.