a lot of small sum stamps

I sent a follow-up of expired card, this time to Germany. Just one of last three follow-ups was received, two still not. Are they stucked in somewhere...?

A few months ago I had a book sending to Europe, put a lot of small sum stamps on the envelope, thought it would be a happy mail. A postal worker said 'it might be stolen because of these beautiful stamps!' and told me to use 'postage certificate' (I don't know what is called in English,) instead of stamps. I had already packed it neatly, decided to let it be and sent.

As a result? It arrived safely!