SKPaymentQueue addTransactionObserver issue when it is added after Amazons TransferManager and a BFTask

I pretty much follow Ray Wenderlich's example for doing In-App purchases. This includes that I get my product information first from a server. In my case I use Amazon Web Services.

The problem I have is that [[SKPaymentQueue defaultQueue] addTransactionObserver:self]; is not working, which means that - (void)paymentQueue:(SKPaymentQueue *)queue updatedTransactions:(NSArray *)transactions is not being called.

[[SKPaymentQueue defaultQueue] addTransactionObserver:self]; gets called within the _productIDCompletionHandler block.

I have narrowed it down to the following code. As long as I call [[SKPaymentQueue defaultQueue] addTransactionObserver:self]; before continueWithExecutor the observer works. If I call the addTransactionObserver within the BFTask block or after, then updatedTransactions method is not being called.

-(void)getProductidsWithCompletionHandler:(AWSAccessProductIDsCompletionHandler)completionHandler {

_productIDCompletionHandler = [completionHandler copy];

AWSS3TransferManager *transferManager = [AWSS3TransferManager defaultS3TransferManager];

// Construct the NSURL for the download location.
NSString *downloadingFilePath = [NSTemporaryDirectory() stringByAppendingPathComponent:@"aws_downloadedproductidfile"];
NSURL *downloadingFileURL = [NSURL fileURLWithPath:downloadingFilePath];

// Construct the download request.
AWSS3TransferManagerDownloadRequest *downloadRequest = [AWSS3TransferManagerDownloadRequest new];

downloadRequest.bucket = [self bucketStringForType:WB_com_test];
downloadRequest.key = [NSString stringWithFormat:@"%@%@",COM_PRODUCTIDS_FILEPATH,COM_PRODUCTIDS_FILENAME];
downloadRequest.downloadingFileURL = downloadingFileURL;

[[transferManager download:downloadRequest] continueWithExecutor:[BFExecutor mainThreadExecutor]
                                                       withBlock:^id(BFTask *task) {
                                                           if (task.error){
                                                               NSLog(@"Error: %@", task.error);
                                                               _productIDCompletionHandler(NO,nil);
                                                               _productIDCompletionHandler = nil;
                                                           }

                                                           if (task.result) {
                                                               NSLog(@"Success: ");

                                                               AWSS3TransferManagerDownloadOutput *output = task.result;
                                                               NSURL *url = (NSURL *)output.body;
                                                               NSArray *products = [NSArray arrayWithContentsOfURL:url];
                                                               DDLogVerbose(@"Success Result Count: %lu", (unsigned long)[products count]);

                                                               if (products)
                                                               {
                                                                   _productIDCompletionHandler( YES, products);
                                                               } else
                                                               {
                                                                   _productIDCompletionHandler( NO, nil);
                                                               }
                                                               _productIDCompletionHandler = nil;
                                                           }
                                                           return nil;
                                                       }];

}

When the above _productIDCompletionHandler handler returns it is on the main thread so this is not the problem, but somehow the thread is "dirty".