purchase method
Purchase a product by its store product ID.
On Android, pass offerToken for subscription offers (base plan tokens).
Returns a PurchaseResult indicating the outcome.
Implementation
Future<PurchaseResult> purchase(String productId,
{String? offerToken}) async {
final result = await _channel.invokeMethod('purchase', {
'productId': productId,
'offerToken': offerToken,
});
return PurchaseResult.fromMap(Map<dynamic, dynamic>.from(result));
}