English term (edited): Do not bend

You have set your term as the simple verb "bend." The simple answer to the question as such would be يثني, but that may or may not help you with your specific context.

You specifically asked if anyone knows the actual phrase used by postal services. I do not know what phrase they use or if they use anything in particular, so I am inventing one.

I would vehemently advise against the monstrous use of the dummy verb لا تقم. Such constructions are simply not Arabic.

The simple prohibition لا تثنِ requires delteting the final long vowel and substituting a diacritic. But I would not recommend it. In Arabic, we do not use a transitive verb unless we are prepared to state the object. If you want to use this construction, then you need to say لا تثنِ المغلَّف

Arabic constructions tend to be more nominal than verbal. That is why I chose this construction. It also has the advantage of not requiring that you state the object, unless you want to.